[MIPS] Add Android.mk file.

Added a Android.mk file to:

    ${TOP}/hardware/ingenic/xb4780/xbomx

This drives the building of the .so files
when building from ${TOP}.

This should have been in previous commit.
On upstream integration this should be
merged with the previous commit.

Signed-off-by: Pete Delaney <piet.delaney@imgtec.com>