libpayload/lpgcc: Turn references to build dir into a variable

Introduce a `$_OBJ` variable, that points to the build directory for
in-tree usage of `lpgcc`. If unset, the default `../build` relative
to the location of `lpgcc` is used.

Change-Id: I35112d7533d69aa51252dd2bceec010a62522403
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47629
Reviewed-by: Raul Rangel <rrangel@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/libpayload/bin/lp.functions

@@ -35,6 +35,10 @@ warn() {
 	echo "Warning: $1"
 }
 
+# For in-tree builds, allow to override the libpayload build dir.
+
+_OBJ=${_OBJ:-$BASE/../build}
+
 # If the user didn't specify LIBPAYLOAD_PREFIX, then preload it
 # with the default prefix value
 
@@ -48,8 +52,8 @@ fi
 
 if [ -f $BASE/../lib/libpayload.a ]; then
 	_LIBDIR=$BASE/../lib
-elif [ -f $BASE/../build/libpayload.a ]; then
-	_LIBDIR=$BASE/../build
+elif [ -f $_OBJ/libpayload.a ]; then
+	_LIBDIR=$_OBJ
 else
 	_LIBDIR=$LIBPAYLOAD_PREFIX/lib
 fi

payloads/libpayload/bin/lpgcc

@@ -137,7 +137,7 @@ while [ $# -gt 0 ]; do
 	shift
 done
 
-_CFLAGS="$_ARCHEXTRA -nostdinc -nostdlib -I$BASE/../build -I$_INCDIR -I$_ARCHINCDIR -D__LIBPAYLOAD__=1"
+_CFLAGS="$_ARCHEXTRA -nostdinc -nostdlib -I$_OBJ -I$_INCDIR -I$_ARCHINCDIR -D__LIBPAYLOAD__=1"
 
 if [ "$CONFIG_LP_PDCURSES" = y ]; then
   _CFLAGS="$_CFLAGS -I$BASE/../curses/PDCurses"
@@ -167,8 +167,8 @@ else
     _LIBGCC=`$DEFAULT_CC $_ARCHEXTRA -print-libgcc-file-name`
     if [ -f $_ARCHLIBDIR/head.o ]; then
 	    HEAD_O=$_ARCHLIBDIR/head.o
-    elif [ -f $BASE/../build/head.o ]; then
-	    HEAD_O=$BASE/../build/head.o
+    elif [ -f $_OBJ/head.o ]; then
+	    HEAD_O=$_OBJ/head.o
     else
 	    echo "Could not find head.o"
 	    exit 1