arm-bsp/u-boot: free float the u-boot bbappend

Since the u-boot recipe will be updated in the near future, decouple the
bbappend from the current version to allow for it to work on both
versions (and keep backward compatible with hardknott).

Change-Id: Ia2447a80868f2e5011bf53fafb939b6a4566b662
Signed-off-by: Jon Mason <jon.mason@arm.com>

meta-arm-bsp/recipes-bsp/u-boot/u-boot_%.bbappend

@@ -1,6 +1,6 @@
 # Machine specific u-boot
 
-FILESEXTRAPATHS_prepend := "${THISDIR}/${BP}:"
+FILESEXTRAPATHS_prepend := "${THISDIR}/${PN}:"
 
 #
 # FVP BASE