manual: add some info on the POST_RSYNC hook

LIBFOO_POST_PATCH_HOOKS += LIBFOO_POST_PATCH_FIXUP

----------------------

Using the +POST_RSYNC+ hook

^^^^^^^^^^^^^^^^^^^^^^^^^^^

The +POST_RSYNC+ hook is run only for packages that use a local source,

either through the +local+ site method or the +OVERRIDE_SRCDIR+

mechanism. In this case, package sources are copied using +rsync+ from

the local location into the buildroot build directory. The +rsync+

command does not copy all files from the source directory, though.

Files belonging to a version control system, like the directories

+.git+, +.hg+, etc. are not copied. For most packages this is

sufficient, but a given package can perform additional actions using

the +POST_RSYNC+ hook.

In principle, the hook can contain any command you want. One specific

use case, though, is the intentional copying of the version control

directory using +rsync+. The +rsync+ command you use in the hook can, among

others, use the following variables:

* +$(SRCDIR)+: the path to the overridden source directory

* +$(@D)+: the path to the build directory