Deprecate weak crash console functions

The default behaviour of the plat_crash_console_xxx functions isn't
obvious to someone that hasn't read all the documentation. As they are
not mandatory, it is unlikely that the code will be checked when doing a
platform port, which may mean that some platforms may not have crash
console support at all.

The idea of this patch is to force platform maintainers to decide how
the crash console has to behave so that the final behaviour isn't
unexpected.

Change-Id: I40b2a7b56c5530c1dcd63eace5bd37ae6335056e
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>