bitbake.conf: rework FILESPATH generation.

bitbake.conf: rework FILESPATH generation.

Rework FILESPATH generation to be done in bitbake.conf, avoiding the
confusion about it being in multiple places.  Adds FILESPATHBASE and
FILESPATHPKG which can be manipulated rather than manipulating FILESPATH
directly.

One usage possibility:

FILESPATHBASE =. "${TOPDIR}/files:"

Which would let me provide a custom busybox config for this build by
copying the defconfig from the openembedded metadata into my
build/files/busybox-1.0/ directory, for example.

Signed-off-by: Chris Larson <clarson@mvista.com>