logging: introduce LOG_DISABLE_FANCY_OUTPUT_FORMATTING to save flash

The new Logger subsys uses a very robust formatter function in minimal
libc: _prf().  This adds up to ~3K flash.  For resource constrained
devices running samples that don't use the extra formatting options,
allow them to select LOG_DISABLE_FANCY_OUTPUT_FORMATTING to revert
back to vprintk.

MCUBOOT is one such sample that has a very limited amount of output
and benefits from the flash savings due to small bootloader partition
sizes.

Signed-off-by: Michael Scott <mike@foundries.io>