include/config_fallbacks.h: change fallback for CONFIG_SYS_PBSIZE

Most of the platforms are using CONFIG_SYS_CBSIZE +
sizeof(CONFIG_SYS_PROMPT) + 16 as their value for CONFIG_SYS_PBSIZE,
so let's adopt this for the fallback value of CONFIG_SYS_PBSIZE.

This will allow us to drop an explicit definition of CONFIG_SYS_PBSIZE
from a large number of platforms.

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