devtool: Compare abspath of both B and S

devtool: Compare abspath of both B and S

Either both or none of the paths must be passed through
os.path.abspath or things like 'A//B', 'A/./B/', and 'A/B/' in S will
cause unintentional mismatches even when B = "${S}".

Using os.path.abspath for both seems more likely to be correct as that
will also handle the case where ${B} != ${S} but the abspaths are
equal.

Signed-off-by: Ola x Nilsson <olani@axis.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>