libpayload/lpgcc: Check for `libpayload.config` for in-tree builds

Instead of checking for an already fully build `libpayload.a`, we check
for the `libpayload.config` which is the actual prerequisite to start
using `lpgcc`. This will allow compilation of payload sources before or
in parallel with the build of `libpayload.a`.

Change-Id: Ic0143fefe33560af8b013ae48bbbe231b3ad46f3
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48938
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Raul Rangel <rrangel@chromium.org>

payloads/libpayload/bin/lp.functions

@@ -52,7 +52,7 @@ fi
 
 if [ -f $BASE/../lib/libpayload.a ]; then
 	_LIBDIR=$BASE/../lib
-elif [ -f $_OBJ/libpayload.a ]; then
+elif [ -f $_OBJ/libpayload.config ]; then
 	_LIBDIR=$_OBJ
 else
 	_LIBDIR=$LIBPAYLOAD_PREFIX/lib