xtensa: support configurable processor configurations

Xtensa is a configurable processor architecture, which allows to define
additional instructions and registers. The required variant specific
information for the toolchain is delivered in an 'overlay' file, which
needs to be 'untarred' to the corresponding directories after the
source is installed and patched.
This patch provides support for binutils, gcc, and gdb with a very
limited changes to the build scripts. These additions are only executed
for the Xtensa architecture and have no effect on other architectures.

[Thomas: rebased on top of the 'arch: improve definition of gcc mtune,
mcpu, etc.' patch, and changed 'Target ABI' to 'Target Architecture
Variant'].

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