[Buildroot] Hash checking infrastructure

[Mike <mikez@OpenPlayer.org>]

A small nit about how the hash checking is
working (or in special cases, not working):

Use case:

When using a package source directory override,
the usual version number field is replaced with
the string 'custom'.

If the package being over-ridden has a hash file
in its Buildroot configuration directory,
the hash checker attempts to check the:
<package name>-custom-<archive type> in the 
download directory.

Reference section 8.11.6:
"... will no longer attempt to download, extract 
and patch the package."

I think the behavior should also include skipping
the hash check.

Mike