Allow building bootloader in-tree

Add the missing variables and rules from staging branch.
This allows building the bootloader using:

  m out/target/product/FP3/emmc_appsboot.mbn

The bootloader also needs to be signed so it's usable.

Issue: FP3-A11#40
Change-Id: I65f4c7ce053dbee3e774db8f37eeaef29aa91860
2 files changed