formfactor: Fix filespath usage

The old way of setting THISDIR in a bbappend clobbers THISDIR for
other layers.  This change makes it use the new way that is done
in dylan.

Signed-off-by: Ian Reinhart Geiser <igeiser@devonit.com>
Signed-off-by: Denys Dmytriyenko <denys@ti.com>

recipes-bsp/formfactor/formfactor_0.0.bbappend

@@ -1,3 +1,2 @@
-THISDIR := "${@os.path.dirname(bb.data.getVar('FILE', d, True))}"
-FILESPATH =. "${@base_set_filespath(["${THISDIR}/${PN}"], d)}:"
-
+PRINC := "${@int(PRINC) + 1}"
+FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"