In the interests of simplifying things, remove P and PN from FILESPATH,
instead relying on the BP and BPN versions which work in 99% of cases.
In any problematic case such as a -native only recipe, either the patch
directory can be renamed or the recipe can set FILESPATH specifically.
(From OE-Core rev: fb359583b659cda643973fa285002aaffb729a51)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>