include/config_fallbacks.h: add default for CONFIG_SYS_CBSIZE

CONFIG_SYS_CBSIZE contains the buffer size for input for the
console. The vast majority of platforms define them to some reasonable
value (256, 512 or 1024 bytes), and it is quite annoying to repeat
this definition for all platforms while it isn't really HW-related.

Therefore, let's provide a sane fallback value in config_fallbacks.h,
so that platforms can rely on it instead of having to explicitly
define it.

We use 1024 when KGDB is enabled, and 256 otherwise, which is what the
majority of the platforms are doing.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Tom Rini <trini@konsulko.com>