mutt: fix build with BR2_PACKAGE_SKELETON_NONE

BR2_PACKAGE_SKELETON_NONE does not provide the /var subdirectory which
breaks symlink creation in that directory. Make sure that /var exists
before creating the symlink.

Fixes:
http://autobuild.buildroot.net/results/32d/32dff0df759baf1120e4f11335e735eb0708b237/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/mutt/mutt.mk

@@ -66,6 +66,7 @@ MUTT_CONF_ENV += \
 MUTT_CONF_OPTS += --with-mailpath=/var/mail
 
 define MUTT_VAR_MAIL
+	mkdir -p $(TARGET_DIR)/var
 	ln -sf /tmp $(TARGET_DIR)/var/mail
 endef
 MUTT_POST_INSTALL_TARGET_HOOKS += MUTT_VAR_MAIL