make: Normalize config options for external trees

make: Normalize config options for external trees

We have four different external trees, and four different naming
conventions for specifying the URL, where they live, and what revision
to check out.

Normalize config options on the following bases:
 - QEMU_UPSTREAM
 - QEMU_TRADITIONAL
 - SEABIOS_UPSTREAM
 - OVMF_UPSTREAM

The following suffixes for all trees:
 - _URL : A remote repository to clone from
 - _REVISION : The revision to check out

And the following suffixes for the qemu trees:
 - _INTREE : The location of an inlined tree (for tarball releases)
 - _LOC : Where to actually look (either a directory or a URL)

The following parameters are still supported for backwards
compatibility:
 - CONFIG_QEMU          (=> QEMU_TRADITIONAL_LOC)
 - QEMU_REMOTE          (=> QEMU_TRADITIONAL_URL)
 - QEMU_TAG             (=> QEMU_TRADITIONAL_REVISION)
 - SEABIOS_UPSTREAM_TAG (=> SEABIOS_UPSTREAM_REVISION)

Signed-off-by: George Dunlap <george.dunlap@eu.citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>