golang: Share build cache across package builds

This also adds a config option GOLANG_BUILD_CACHE_DIR to customize the
location of the build cache directory.

Signed-off-by: Jeffery To <jeffery.to@gmail.com>

utils/ctop/Makefile

@@ -35,12 +35,11 @@ endef
 MAKE_PATH:=$(GO_PKG_WORK_DIR_NAME)/build/src/$(GO_PKG)
 MAKE_VARS += \
 	GOPATH=$(GO_PKG_BUILD_DIR) \
-	GOCACHE=$(GO_PKG_CACHE_DIR) \
-	GOTMPDIR=$(GO_PKG_TMP_DIR) \
+	GOCACHE=$(GO_BUILD_CACHE_DIR) \
 	GOROOT_FINAL=$(GO_TARGET_ROOT) \
 	CC=$(TARGET_CC) \
 	CXX=$(TARGET_CXX) \
-	$(call GoPackage/Environment)
+	$(GO_PKG_VARS)
 MAKE_FLAGS += \
 	COMMIT=$(PKG_SOURCE_VERSION)