python: do not replace ccache in the middle of a path

python: do not replace ccache in the middle of a path

Python recipe did a sed s/ccache/$(CCACHE) on the Makefile, which
replaces all "ccache" including ones that consist of a full path.
This leads to build error when building in a project path with
"ccache" in its name. Fix it by only replacing "ccache " with
"$(CCACHE) ".

Signed-off-by: Lei Liu <lei.liu2@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>