kernel: speed-up builds with ccache

Enable ccache for in-tree kernel builds if USE_CCACHE environment variable
is defined to anything but 'false'.

BUG=27447472
Change-Id: Ie32858fface7f05e1dd56bfbef1e19da17e5d6fc
Signed-off-by: Mihai Serban <mihai.serban@intel.com>
1 file changed