app: aboot: Optimize loading flow of bootimage from mmc
app: aboot: Optimize loading flow of bootimage from mmc

Currently we have different loading flow for signed and unsigned
kernel in boot_linux_from_mmc(). However, the main process is
similar. So remove the redundant codes to make the logic more
clear.

Change-Id: I18680a497ac03ce0914853794b65858acb2822d5

efidroid/lk,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,efidroid/lk