arch/arch.mk.xtensa: relax check on overlay file to apply only to internal toolchains

Commit 4cbf7336 ("arch/xtensa: custom
configuration requires an overlay") added a check in
arch/arch.mk.xtensa to bail out if a custom Xtensa core is selected
but not overlay file is provided. While this is indeed a perfectly
valid check to make when building an internal toolchain, with an
external toolchain it's entirely possible to build with no overlay
file: the toolchain already exists, and there's no overlay to be
applied in the context of the Buildroot build.

And indeed commit 4cbf7336 broke some
of the runtime test cases that use a custom Xtensa core configuration,
with no overlay, to test the toolchains.bootlin.com Xtensa external
toolchain. By relaxing the check to only apply to internal toolchain
configurations, we fix those test cases.

It is to be noted that this still allows a configuration where gdb gets
built for a custom core, but with no overlay, so basically that means
the fsf variant, which can lead to build or run failures that
4cbf7336 attempted to fix to begin with. This still covers the
most common cases.

Finally, it also means being able to build a kernel with no overlay, but
this is offset by the fact that the kernel may be already patched with
an overlay (as it is possible to specify a custom kernel), which is most
probably what people using a custom core would have.

Fixes:

  https://gitlab.com/buildroot.org/buildroot/-/jobs/3282261966
  https://gitlab.com/buildroot.org/buildroot/-/jobs/3282261963



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr: extend commit log with last two paragraphs]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>