use ${} with make vars instead of $()
use ${} with make vars instead of $()

BUG=None
TEST=make still builds
BRANCH=none

Change-Id: Ie7292e4cc88338dc6544fd859028ce55557578b4
Reviewed-on: https://chromium-review.googlesource.com/241043
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Reviewed-by: Bill Richardson <129945214b1d548d8e49b6c29c43094f8c78057f@google.com>

coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,coreboot/vboot,coreboot/vboot,coreboot/vboot,geekboxzone/mmallow_external_vboot_reference,acorn-marvell/vboot_reference,acorn-marvell/vboot_reference,acorn-marvell/vboot_reference