package/libcamera: Prevent builds on m68k

The ControlValue structure is currently defined with a 16-bit hole
(causing unaligned access to the numElements_ field, though that's a
separate topic).

This structure has a static assertion to ensure that its size does not
change without due care, as it forms part of our ABI and is used in
Serialisation between the pipeline handlers and IPA components.

The m68k architecture is the only target which fails this assertion,
which is likely because it can pack the structure more efficiently,
producing a different binary size.

This is likely an area we will tackle before stabilising our ABI, but
until then, disable m68k builds as libcamera is not expected to be
supported on this target.

Fixes;
  - http://autobuild.buildroot.net/results/9dce26e94299a2c61bba60cbc7803926e2f85e29/



Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
[yann.morin.1998@free.fr, suggestions from Thomas:
  - introduce BR2_PACKAGE_LIBCAMERA_ARCH_SUPPORTS
  - propagate that to the comment
  - add autobuilder reference
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>