cbfstool: Build vboot library

Currently cbfstool cherry-picks a few files from vboot and hopes these
files will work standalone without any dependencies. This is pretty
brittle (for example, CL:2084062 will break it), and could be improved
by building the whole vboot library and then linking against it.
Therefore, this patch creates a new target $(VBOOT_HOSTLIB) and includes
it as a dependency for cbfstool and ifittool.

To prevent building the vboot lib twice (one for cbfstool and the other
for futility) when building coreboot tools together, add the variable
'VBOOT_BUILD' in Makefile to define a shared build path among different
tools so that vboot files don't need to be recompiled.

Also ignore *.o.d and *.a for vboot library.

BRANCH=none
BUG=none
TEST=make -C util/cbfstool
TEST=make -C util/futility
TEST=Run 'make tools' and make sure common files such as 2sha1.c are
     compiled only once
TEST=emerge-nami coreboot-utils

Change-Id: Ifc826896d895f53d69ea559a88f75672c2ec3146
Signed-off-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39390
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Makefile

@@ -42,6 +42,8 @@ objutil ?= $(obj)/util
 objk := $(objutil)/kconfig
 absobj := $(abspath $(obj))
 
+VBOOT_HOST_BUILD ?= $(abspath $(objutil)/vboot_lib)
+
 COREBOOT_EXPORTS := COREBOOT_EXPORTS
 COREBOOT_EXPORTS += top src srck obj objutil objk