build: fix target header overlay

LOCAL_C_INCLUDES as defined by the makefile should be put AFTER
the overlay includes so the overlay always takes precedence.

Change-Id: I489b2aab6dbacd9122d834f85e07b63ed1271f07
kernel.mk: Fix darwin regression caused by 732db8b1.

Change-Id: I1d9c2e231eca695ff6292ce5e163580cfd3fbb86