Makefile: use correct md5 extension

* our servers and OTA uses md5 extensions not md5sum

Change-Id: I404349a3d6b48d83f45776a749f6c5a84565a047
Signed-off-by: houst0nn <houstonn@gmail.com>
Merge remote-tracking branch 'PA-build/kitkat' into kitkat
Merge branch 'kitkat' of github.com:AOSPAL/android_build into kitkat