mka: allow mka to be run from anywhere in the tree

The Android build system expects to be launched from the top of the
tree. Invoking mka at any other level often has unintended consequence
(e.g. targets the host ARCH because ARCH was not set).  The various
build shortcuts like m and mm invoke Make from the top of the tree. mka
now does the same.

Change-Id: Id956cf04cbaf1d12d12549ca54e32b9c2ce1fc29