package: add configuration option to specify a local override file

The user can now create a custom local override file to override the
source directory for various packages.

An example override file:

 ZLIB_OVERRIDE_SRCDIR = /tmp/zlib
 STRACE_OVERRIDE_SRCDIR = /opt/strace-4.5.20

would tell Buildroot to use the zlib and strace source code from the
specified directories, instead of download, extracting and patching
the code has done usually by Buildroot.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>