base.bbclass: Fix way to check ccache path

The previous code had 2 issues:
1. make hosttools/ccache always link to host's ccache (/usr/bin/ccache)
even we have one buildtools
2. make hosttools/gcc etc, link to host's gcc event we have one
buildtools when keyword ccache in buildtools's path, eg:
/mnt/ccache/bin/buildtools

This patch is for fix above issues.

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>