app: aboot: Fix splash partition corruption

The first 4k bytes ( a page) of the splash partition is reserved
for storing display DFPS PLL codes. When PLL codes are written to
the splash partition, it corrupts the splash image header. To fix this,
splash image is programmed only after the PLL codes offset in the
splash partition.

Change-Id: I50df8d4c87d03065c775e76c83da892dba9c5efc