config_fallbacks: Add a default entry for CONFIG_SYS_PBSIZE

Entering the maximum number of characters defined by CONFIG_SYS_CBSIZE into
the console and hitting enter afterwards, causes a hang in the system because
CONFIG_SYS_PBSIZE is not capable of storing the characters of the error message:
"Unknown command '' - try 'help'".

Provide a default size for CONFIG_SYS_PBSIZE so that it can store the error
message and allows the error message to be printed correctly with no hang.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>