arch/xtensa: allow specifying path to tarball file

currently, specifying a custom Xtrensa core is done with two variables:
  - the core name
  - the directory containing the overlay tarball

However, the core name only serves to construct the tarball name, and is
not used whatsoever to configure any of the toolchain components
(binutils, gcc or gdb), except through the files that are overlayed in
their respective source trees.

This has two main drawbacks:
  - the overlay file must be named after the core,
  - the tarball can not be compressed.

Furthermore, it also makes it extremely complex to implement a download
of that tarball.

So, those two variables can be squeezed into a single variable, that is
the complete path of the overlay tarball.

Update the qemu-xtensa defconfig accordingly.

Note: we do not add a legacy entry for BR2_XTENSA_CORE_NAME, since it
was previously a blind option in the last release, and there's been no
release since we removed BR2_XTENSA_CUSTOM_NAME. So, we just update the
legacy comments for BR2_XTENSA_CUSTOM_NAME, since that's all the user
could have seen in any of our releases so far.

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