Store the value from BOARD_CUSTOM_BOOTIMG_MK to a file

This is needed so it can be read in when re-signing a package to
use the proper boot image.

Print notice when using a custom bootimage makefile

Change-Id: I6baab9f8964ce5d4fa9487f2859af9c385d7b21b
releasetools: don't create prebuilt_dir path if it exits

This path may already exists from previous build tasks. Attempting
to create it again will cause an error.

Change-Id: I4baad8bc846470b99f6ce9207c0682401406decc
releasetools: use variable instead of fixed name for bootable image

When building for a device with a custom bootimage makefile, this
would cause the boot image to be used as the recovery image during
fastboot zip creation.

Change-Id: I075cfe4e9a2a8384d3767de351ec7f751f3e95ab