softethervpn: Pass down CFLAGS, CPPFLAGS and LDFLAGS

The softethervpn does not actually supported CCFLAGS, or other standard
variables, so we need to override CC to include all of those to fix
build errors with external toolchains.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>

net/softethervpn/Makefile

@@ -12,7 +12,7 @@ PKG_NAME:=softethervpn
 PKG_VERSION:=4.22-9634
 PKG_VERREL:=beta
 PKG_VERDATE:=2016.11.27
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 
 PKG_SOURCE:=softether-src-v$(PKG_VERSION)-$(PKG_VERREL).tar.gz
 PKG_SOURCE_URL:=http://www.softether-download.com/files/softether/v$(PKG_VERSION)-$(PKG_VERREL)-$(PKG_VERDATE)-tree/Source_Code/
@@ -78,8 +78,8 @@ MAKE_FLAGS += \
 	-f src/makefiles/linux_$(if $(CONFIG_ARCH_64BIT),64,32)bit.mak
 
 # Map nonstandard CCFLAGS variable to standard TARGET_CFLAGS
-MAKE_VARS += \
-	CCFLAGS="$(TARGET_CFLAGS)"
+MAKE_FLAGS += \
+	CC="$(TARGET_CC) $(TARGET_CFLAGS) $(TARGET_CPPFLAGS) $(TARGET_LDFLAGS)"
 
 define Build/Configure
 	# Fetch prebuilt hamcore.se2 from staging dir