mkbootimg: Support custom mkbootimg implementation

Summary:
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.
Signed-off-by: Bjorn Andersson <bjorn.andersson@sonymobile.com>

Test Plan: win

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D377
improve how strict aliasing is disabled

Summary:
Strict aliasing may now be disabled locally
by setting LOCAL_NO_STRICT_ALIASING_SUPPORT, or globally
by setting BUILD_DISABLE_STRICT_ALIASING.

Test Plan: win

Reviewers: randall.rushing

Differential Revision: http://review.codefirex.com/D378