mkbootimg: Support custom mkbootimg implementation

Support using custom mkbootimg to allow boards to specify custom
boot image formats. Also export this as the environment variable
MKBOOTIMG to the *_from_target_files releasetools scripts.

Change-Id: I2084273b1175de097fb7da5c4f2264ea8014d74f
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>