quilt.mk: don't error on refresh/update if patches doesn't exist

The current code fails if we have package or host tools with no patches
to apply. The error printend is the following: (taking ubus as an
example)

make[2]: Entering directory '/home/ansuel/openwrt-ansuel/openwrt/scripts/config'
make[2]: 'conf' is up to date.
make[2]: Leaving directory '/home/ansuel/openwrt-ansuel/openwrt/scripts/config'
make[1]: Entering directory '/home/ansuel/openwrt-ansuel/openwrt'
make[2]: Entering directory '/home/ansuel/openwrt-ansuel/openwrt/package/system/ubus'
The source directory contains no quilt patches.
make[2]: *** [Makefile:81: quilt-check] Error 1
make[2]: Leaving directory '/home/ansuel/openwrt-ansuel/openwrt/package/system/ubus'
time: package/system/ubus/refresh#0.06#0.00#0.07
    ERROR: package/system/ubus failed to build.
make[1]: *** [package/Makefile:120: package/system/ubus/refresh] Error 1
make[1]: Leaving directory '/home/ansuel/openwrt-ansuel/openwrt'
make: *** [/home/ansuel/openwrt-ansuel/openwrt/include/toplevel.mk:232: package/ubus/refresh] Error 2

This error is wrong and actually caused by quilt-check trying to find
patches while in reality the package doesn't have any patch to refresh.

To better handle this case, introduce a fake Quilt define template that
just print a warning message indicating the user that the refresh/update
is skipped and exit succesfully and use these fake define only if we
detect that the patches directory is empty or non existent.

Example:
make[2]: Entering directory '/home/ansuel/openwrt-ansuel/openwrt/scripts/config'
make[2]: 'conf' is up to date.
make[2]: Leaving directory '/home/ansuel/openwrt-ansuel/openwrt/scripts/config'
make[1]: Entering directory '/home/ansuel/openwrt-ansuel/openwrt'
make[2]: Entering directory '/home/ansuel/openwrt-ansuel/openwrt/package/system/ubus'
Skipping refresh. /home/ansuel/openwrt-ansuel/openwrt/package/system/ubus/patches doesn't exist or is empty.
make[2]: Leaving directory '/home/ansuel/openwrt-ansuel/openwrt/package/system/ubus'
time: package/system/ubus/refresh#0.05#0.00#0.07
make[1]: Leaving directory '/home/ansuel/openwrt-ansuel/openwrt'

This is needed to support run like package/refresh that will run the
refresh process on any package present in the buildroot.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

include/quilt.mk

@@ -175,7 +175,19 @@ define Quilt/Template
 	$(Quilt/Refresh/$(4))
 endef
 
-Build/Quilt=$(call Quilt/Template,$(PKG_BUILD_DIR),,,$(if $(TARGET_BUILD),Kernel,Package))
-Host/Quilt=$(call Quilt/Template,$(HOST_BUILD_DIR),HOST_,host-,Host)
+define Quilt/SkipTemplate
+  $(3)refresh:
+	@echo "Skipping refresh. $(1) doesn't exist or is empty".
+
+  $(3)update:
+	@echo "Skipping update. $(1) doesn't exist or is empty".
+endef
+
+Build/Quilt=$(if $(wildcard $(PATCH_DIR)/.*), \
+	$(call Quilt/Template,$(PKG_BUILD_DIR),,,$(if $(TARGET_BUILD),Kernel,Package)), \
+	$(call Quilt/SkipTemplate,$(PATCH_DIR)))
+Host/Quilt=$(if $(wildcard $(HOST_PATCH_DIR)/.*), \
+	$(call Quilt/Template,$(HOST_BUILD_DIR),HOST_,host-,Host), \
+	$(call Quilt/SkipTemplate,$(HOST_PATCH_DIR)))
 
 endif