golang: Add the ability to custom build environment

by overwrite the GoPackage/Environment definition

Signed-off-by: Xingwang Liao <kuoruan@gmail.com>

lang/golang/golang-package.mk

@@ -147,7 +147,7 @@ define GoPackage/GoSubMenu
   CATEGORY:=Languages
 endef
 
-define GoPackage/Environment
+define GoPackage/Environment/Default
 	GOOS=$(GO_OS) \
 	GOARCH=$(GO_ARCH) \
 	GO386=$(GO_386) \
@@ -160,6 +160,8 @@ define GoPackage/Environment
 	CGO_CXXFLAGS="$(filter-out $(GO_CFLAGS_TO_REMOVE),$(TARGET_CXXFLAGS))"
 endef
 
+GoPackage/Environment=$(call GoPackage/Environment/Default,)
+
 # false if directory does not exist
 GoPackage/is_dir_not_empty=$$$$($(FIND) $(1) -maxdepth 0 -type d \! -empty 2>/dev/null)