Disable TARGET_BOOTIMG_SIGNED
AppsBL: In PL1.2 the boot.img is not signed. This patch disables the TARGET_BOOTIMG_SIGNED to skip boot.img authentication step in Apps Boot loader.

Change-Id: I50c21a6b6fbe60277a8a03b72e47b14f5d93cbfa
Merge "Disable TARGET_BOOTIMG_SIGNED AppsBL: In PL1.2 the boot.img is not signed. This patch disables the TARGET_BOOTIMG_SIGNED to skip boot.img authentication step in Apps Boot loader."