libpayload: Use top level xcompile

Instead of having a second copy that already within 2-3 days
becamer quite outdated, use the same xcompile copy for coreboot
and libpayload, as we do with Kconfig already.

This requires a simple change to the top level xcompile to understand
both CONFIG_COMPILER_GCC and CONFIG_LP_COMPILER_GCC (only one of
them will occur at the same time)

libpayload's .xcompile target was moved later so that it can make use
of $(top)

Change-Id: I44001067f551cd0776dd303cbaeaa40eb3d5c1db
Signed-off-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-on: http://review.coreboot.org/10863
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

util/xcompile/xcompile

@@ -174,7 +174,7 @@ report_arch_toolchain() {
 # elf${TWIDTH}-${TBFDARCH} toolchain (${GCC})
 ARCH_SUPPORTED+=${TARCH}
 SUBARCH_SUPPORTED+=${TSUPP-${TARCH}}
-ifeq (\$(CONFIG_COMPILER_GCC),y)
+ifeq (\$(CONFIG_COMPILER_GCC)\$(CONFIG_LP_COMPILER_GCC),y)
 CC_${TARCH}:=${GCC}
 CFLAGS_${TARCH}:=${CFLAGS_GCC}
 COMPILER_RT_${TARCH}:=${CC_RT_GCC}