Implement fast loading in Android bootmethod

Right now when bootmeth_android.c reads the boot.img from the boot_a partition, it reads the whole partition size.

This is wasteful, since we should only read the "useful" bytes (shorter) from the boot_a partition.

Roman proposed something for the abootimg command here: https://patchwork.ozlabs.org/project/uboot/patch/20240519191856.2582174-4-r.stratiienko@gmail.com/

Implement something similar for bootmeth_android.c

added android label

Done via here: https://patchwork.ozlabs.org/project/uboot/patch/20241121-bootmeth-android-part-sizes-v1-1-25760bbd0f08@baylibre.com/

Implemented via: https://patchwork.ozlabs.org/project/uboot/patch/87plmip1m6.fsf@baylibre.com/