Revert "kernel: Use CPATH for including host headers"

This reverts commit 2baeb4fb.

CPATH is used for both host and target. Meaning that system headers
will be used for target compilation. It is also a higher priority than
isystem, which is used for the compiler include paths, making this
override the compiler headers.

Change-Id: I4d836698f25d2d97d917d323a22ee359b937fcb4