Makefile: fix build break in sdk target

After commit 6729050f3a49dba42d2d881d31dbefd6b82b8dad nothing creates
$(HOST_DIR)/share/buildroot anymore, causing sdk to fail with:

 /bin/bash: .../output/host/share/buildroot/sdk-location: No such file or directory

Add creation of that directory to the "sdk" build steps itself.

Signed-off-by: Stefan Becker <chemobejk@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

Makefile

@@ -574,6 +574,7 @@ sdk: world
 	$(TOPDIR)/support/scripts/fix-rpath host
 	$(TOPDIR)/support/scripts/fix-rpath staging
 	$(INSTALL) -m 755 $(TOPDIR)/support/misc/relocate-sdk.sh $(HOST_DIR)/relocate-sdk.sh
+	mkdir -p $(HOST_DIR)/share/buildroot
 	echo $(HOST_DIR) > $(HOST_DIR)/share/buildroot/sdk-location
 
 # Populating the staging with the base directories is handled by the skeleton package