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

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

We exit 1 after saying that there are no patches because later in the
function quilt pop fails to execute.

Having no patches for a package and calling refresh should not be
a critical error and the function should just do nothing.

To handle this improve quilt.mk with the following addition.
- If we don't have any patch for the package, we print a warning and we
  create an empty series. This is useful to trick quilt and make it do
  nothing.
  We also create a status file .quilt_no_patch to detect in the other
  function that we don't have patches to handle.
- In refresh makefile target, we check if .quilt_no_patch exist and
  we skip quilt cleanup if this exist.
- In RefreshDir function we change the logic and now we delete the
  patches directory and not only the content. This is done as a cleanup
  to clean case with empty patches directory.
- In RefreshDir we check if .quilt_no_patch exist and we skip creating
  the patches directory and copying the refreshed patches.
- In RefreshDir we delete at the end any trace of .quilt_no_patch if
  present.

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>