spl: Split sprintf, strto* from SPL serial in Kconfig

When SPL serial is disabled, callers who need sprintf or strtoul fail
because their inclusion is guarded by CONFIG_SPL_SERIAL_SUPPORT/
CONFIG_TPL_SERIAL_SUPPORT.

Split printf, sprintf and strto into their own entries and then select
all of them if SERIAL_SUPPORT is enabled to match the current behaviour.

Include panic.o unconditionally as it can be called from anywhere which
uses BUG_ON().

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>