arch/xtensa: add definitions for the Xtensa overlay

arch/xtensa: add definitions for the Xtensa overlay

Currently, we have four packages that need to use the Xtensa overlay:
binutils, gcc-initial, gcc-final and gdb. However, they all use three
different ways to check for, and use the overlay. Furthermore, two of
them define the same variable: XTENSA_CORE_NAME is defined both by
binutils and gdb

In order to sanitise that, have the Xtensa architecture export the
appropriate variables: the name of the core and the path to the overlay
tarball. Packages can then check and use those variables.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>