[x86] Android 4.4.4 for x86 needs new implementation of "security_compute_relabel"

Change-Id: I2e15f58f11c9ee74ed8e1b7bf731efd881b890d9
busybox: Use 'gsed' in Mac builds

Mac builds were broken because its sed implementation is not compatible
with some of the features used in this script. We already require gsed
for the Android build on Mac so this changes the script to take that
into account.

Change-Id: I37c10f8c543bdaf2c84409dacfaeccd24eb152d7
busybox: restore mac/linux bash compat.

previous commit was merged too fast.

Change-Id: I2573ce6074e988bae0cd2157fcdf1269f6b1ac99