package/pkg-cargo: convert to uppercase RUSTC_TARGET_NAME

RUSTC_TARGET_NAME's dashes are substituted with underscores, but this is
not enough to fix building, we also need to make it UPPERCASE. So let make
it like that by using UPPERCASE macro.

Fixes:
http://autobuild.buildroot.net/results/498/498b96224c8bcff91542ca3731dfd66891f4ec23/

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
[Arnout: remove subst, implied by UPPERCASE]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/pkg-cargo.mk

@@ -44,7 +44,7 @@ PKG_CARGO_ENV = \
 	CARGO_UNSTABLE_TARGET_APPLIES_TO_HOST="true" \
 	CARGO_TARGET_APPLIES_TO_HOST="false" \
 	CARGO_BUILD_TARGET="$(RUSTC_TARGET_NAME)" \
-	CARGO_TARGET_$(subst -,_,$(RUSTC_TARGET_NAME))_LINKER=$(notdir $(TARGET_CROSS))gcc
+	CARGO_TARGET_$(call UPPERCASE,$(RUSTC_TARGET_NAME))_LINKER=$(notdir $(TARGET_CROSS))gcc
 
 HOST_PKG_CARGO_ENV = \
 	$(PKG_COMMON_CARGO_ENV)