Add separate directory for postinstall intercepts

Add separate directory for postinstall intercepts

The scripts/postinst-intercepts will contain all postinstall hooks that
we need to run after all packages have been installed.

If one wants to install such a postinst hook, all it needs to do is put
the hook in this directory and, from the package postinstall scriptlet,
call:
	postinst_intercept <hook_name> <package_name> <var1=...> ...

This will, practically, add the package_name in the list of packages
that need the hook to run and, also, set any variables that would be
needed in the hook. For example, variables like ${libdir}, ${bindir},
etc. that might depend on distribution can be passed on to the script in
this way.

(From OE-Core rev: 0ef538d75c2f3921a2fcbe6ca1deed5525b276cc)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>