== Parted ==
  parted /dev/sdX
  mklabel msdos
  mkpart primary fat32 2048s 150M
  mkpart primary ext4 150M -1
  
  mkfs.vfat -F 32 /dev/sdX1
  mkfs.ext4 -L root -m 0.1 /dev/sdX2

== Understand the firmware ==
When I open the folder **install/** into **cl-som-imx7-linux.zip**, I scan with ''file'' command to understand what I have, usually ARM hardware use uboot, then I hope uboot system + linux kernel + ramdisk.

I wish //gentoo// to have better 0-days hardened, but have to uSD card allow me debug the OS offline, save the data in all case.

  * boot.scr:                   u-boot legacy uImage, , Linux/ARM, Script File (Not compressed), 1086 bytes, Mon Aug 21 06:43:51 2017, Load Address: 0x00000000, Entry Point: 0x00000000, Header CRC: 0xF57D03AB, Data CRC: 0x289863A9
  * imx7d-cl-som-imx7-m4.dtb:   Device Tree Blob version 17, size=38386, boot CPU=0, string block size=2350, DT structure block size=35980
  * imx7d-cl-som-imx7.dtb:      Device Tree Blob version 17, size=38270, boot CPU=0, string block size=2330, DT structure block size=35884
  * imx7d-sbc-imx7-csi.dtb:     Device Tree Blob version 17, size=43079, boot CPU=0, string block size=2679, DT structure block size=40344
  * imx7d-sbc-imx7-lvds.dtb:    Device Tree Blob version 17, size=42667, boot CPU=0, string block size=2667, DT structure block size=39944
  * imx7d-sbc-imx7-m4.dtb:      Device Tree Blob version 17, size=41964, boot CPU=0, string block size=2600, DT structure block size=39308
  * imx7d-sbc-imx7.dtb:         Device Tree Blob version 17, size=41848, boot CPU=0, string block size=2580, DT structure block size=39212
  * imx7d-sbc-iot-imx7-can.dtb: Device Tree Blob version 17, size=42078, boot CPU=0, string block size=2542, DT structure block size=39480
  * imx7d-sbc-iot-imx7.dtb:     Device Tree Blob version 17, size=41978, boot CPU=0, string block size=2542, DT structure block size=39380
  * imx7d-sbc-ucm-imx7.dtb:     Device Tree Blob version 17, size=41205, boot CPU=0, string block size=2565, DT structure block size=38584
  * imx7d-ucm-imx7.dtb:         Device Tree Blob version 17, size=37249, boot CPU=0, string block size=2277, DT structure block size=34916
  * install.ext2:               Linux rev 1.0 ext2 filesystem data, UUID=dba7e302-51ee-45c2-a182-25674c4846dc, volume name "installer.imx7"
  * kernel.img:                 Linux kernel ARM boot executable zImage (little-endian)
  * ramdisk.img:                u-boot legacy uImage, , Linux/ARM, RAMDisk Image (gzip), 6561285 bytes, Sun Aug 20 14:26:44 2017, Load Address: 0x00000000, Entry Point: 0x00000000, Header CRC: 0x804BB927, Data CRC: 0x4472DA7C
  * rootfs.tar.bz2:             bzip2 compressed data, block size = 900k
  * zImage:                     Linux kernel ARM boot executable zImage (little-endian)

Into the boot scr decompiled I see:
  setenv bootargs "root=/dev/ram0 rw ramdisk_size=16384 console=$tty vram=16M auto_install ${board_param}"
  setenv ramimg ramdisk.img
  load ${storagetype} ${storagedev} ${fdtaddr} ${fdtfile}
  load ${storagetype} ${storagedev} ${loadaddr} ${kernel}
  load ${storagetype} ${storagedev} ${rdaddr} ${ramimg}

Then it will be easy the boot the current kernel to uSD part to boot gentoo.

I do:
  dd bs=1 skip=72 if=boot.scr of=boot.script
I change into:
  #System for CL-SOM-iMX7
  
  # Parameters
  setenv tty "ttymxc0,115200"
  setenv mmcdev 0
  setenv rdaddr 0x82000000
  setenv loadaddr 0x80800000
  setenv fdtaddr 0x83000000
  setenv fdt_high 0xffffffff
  setenv initrd_high 0xffffffff
  
  test -z "$fdtfile" && setenv fdtfile $fdt_file
  
  setenv bootargs "root=/dev/mmcblk0p2 ro console=$tty vram=16M"
  setenv kernel kernel.img
  
  test -z "$storagetype" && setenv storagetype mmc
  test -z "$storagedev" && setenv storagedev ${mmcdev}
  
  # Script start
  load ${storagetype} ${storagedev} ${fdtaddr} ${fdtfile}
  load ${storagetype} ${storagedev} ${loadaddr} ${kernel}
  echo "Starting uSD system ..."
  bootz ${loadaddr} ${fdtaddr}
  
After install the package **dev-embedded/u-boot-tools** I do:
  mkimage -C none -A arm -T script -d boot.script boot.scr

After I do my ext4 part with rootfs.tar.bz2 content into my second uSD partition. Or other system keeping the **/lib/modules/** at the first time util I do my own kernel. FAT32 on first part with:
  * boot.scr
  * kernel.img
  * imx7d-sbc-iot-imx7.dtb