trusted-firmware-a.mk: pass package version as version identifier

Upon boot it now prints:
NOTICE:  BL1: v2.4(release):OpenWRT v2.4-1 (espressobin-v3-v5-1gb-2cs) (Marvell-devel-18.12.0)

Signed-off-by: Andre Heider <[email protected]>

include/trusted-firmware-a.mk

@@ -73,6 +73,7 @@ define Build/Compile/Trusted-Firmware-A
 		CROSS_COMPILE=$(TARGET_CROSS) \
 		OPENSSL_DIR=$(STAGING_DIR_HOST) \
 		PLAT=$(PLAT) \
+		BUILD_STRING="OpenWRT v$(PKG_VERSION)-$(PKG_RELEASE) ($(VARIANT))" \
 		$(TFA_MAKE_FLAGS)
 endef