libpayload: Install vboot headers and add include paths to lpgcc

New CBFS API uses commonlib/bsd/cbfs_serialized.h, which includes
vboot's vb2_sha.h. And, because vboot's includes are not available
in libpayload's installation directory nor in lpgcc paths, it was
causing compilation errors. This patch fixes this issue.

lpgcc will look for `vboot` directory like it is doing for `include`
directory to create correct paths. However, if payload will be built
using libpayload's build dir as a base, then vboot headers from
3rdparty/vboot will be used, as there is no way to pass VBOOT_SOURCE
from makefile to lpgcc.

Moreover, this patch moves VBOOT_SOURCE to the main Makefile to
make it available for installation target, to install headers from
vboot directory provided by caller.

Change-Id: I68dd7e1545cfcaf24547d8a9fe289447c79da222
Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Reported-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/61032
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/bin/lp.functions

@@ -63,3 +63,9 @@ if [ -d $BASE/../include ]; then
 else
 	_INCDIR=$LIBPAYLOAD_PREFIX/include
 fi
+
+if [ -d $BASE/../vboot ]; then
+	_VBOOTINCDIR=$BASE/../vboot/include
+else
+	_VBOOTINCDIR=$LIBPAYLOAD_PREFIX/../vboot/include
+fi