util/cbmem: Provide a way to override coreboot path

Right now cbmem uses a fix path to reach coreboot src path (../../).
This makes it impossible to compile cbmem out of the coreboot tree (e.g.
copy just the cbmem directory elsewhere and compile).

This patch adapts the technique from cbfstool and adds a variable called
'TOP' which points to coreboot root directory and which can be
overridden at build time by providing it to make as an argument. This
will enable a stand-alone build of cbmem.

Change-Id: I2732f75310e10716e5aa74e094e0bf628ad22f0b
Signed-off-by: Werner Zeh <werner.zeh@siemens.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/69686
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>

util/cbmem/Makefile

@@ -2,7 +2,8 @@
 ## SPDX-License-Identifier: GPL-2.0-only
 
 PROGRAM   = cbmem
-ROOT      = ../../src
+TOP      ?= $(abspath ../..)
+ROOT      = $(TOP)/src
 CC       ?= $(CROSS_COMPILE)gcc
 INSTALL  ?= /usr/bin/env install
 PREFIX   ?= /usr/local