cmd: abootimg: Add abootimg command

This command can be used to extract fields and image payloads from
Android Boot Image. It can be used for example to implement boot flow
where dtb is taken from boot.img (as v2 incorporated dtb inside of
boot.img). Using this command, one can obtain needed dtb blob from
boot.img in scripting manner, and then apply needed dtbo's (from "dtbo"
partition) on top of that, providing then the resulting image to bootm
command in order to boot the Android.

Also right now this command has the sub-command to get an address and
size of recovery dtbo from recovery image (for non-A/B devices only,
see [1,2] for details).

It can be tested like this:

    => mmc dev 1
    => part start mmc 1 boot_a boot_start
    => part size mmc 1 boot_a boot_size
    => mmc read $loadaddr $boot_start $boot_size
    => abootimg get ver
    => abootimg dump dtb

[1] https://source.android.com/devices/bootloader/boot-image-header
[2] https://source.android.com/devices/architecture/dto/partitions



Signed-off-by: Sam Protsenko <joe.skb7@gmail.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>