core/Makefile: update BOARD_CUSTOM_BOOTIMG_MK fix
this updates code left out from commit fd88ca3

Change-Id: I86ceb22001b92f1b631000d0cefa84a64d11370e
build: Fix usage of releasetool

Change-Id: I15766759195e86c0f9395cfce0b98c5a5a3f6f63