toolchain: fix multilib symlink for external toolchains

ARCH_SUBDIR is a shell variable, so it should be referenced with
$${ARCH_SUBDIR}. Without this, no symbolic link is created, and the
external toolchain fails to work if the non-default multilib variant
is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

toolchain/helpers.mk

@@ -96,7 +96,7 @@ copy_toolchain_sysroot = \
 		if [ ! -d $${ARCH_SYSROOT_DIR}/usr/include ] ; then \
 			cp -a $${SYSROOT_DIR}/usr/include $(STAGING_DIR)/usr ; \
 		fi ; \
-		ln -s . $(STAGING_DIR)/$(ARCH_SUBDIR) ; \
+		ln -s . $(STAGING_DIR)/$${ARCH_SUBDIR} ; \
 	fi ; \
 	find $(STAGING_DIR) -type d | xargs chmod 755