app: aboot: Use block size while reading from splash partition
app: aboot: Use block size while reading from splash partition

Reading from splash partition assumes block size for emmc. Make
the api generic to get the block size from storage device and use
it.

Change-Id: I2830191a62acb945e85a1d30279baa008d28e38d

efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,CanarySolutions/little-kernel,CanarySolutions/little-kernel,efidroid/lk,CanarySolutions/little-kernel,CanarySolutions/little-kernel,efidroid/lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk,CanarySolutions/little-kernel,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,Foxda-Tech/polaris-bootable-bootloader-lk,efidroid/lk