logging: kconfig: Remove legacy SYS_LOG symbols

Replaced by the new CONFIG_LOG system.

Also remove the "Logging Options" menu and turn the LOG symbol into a
'menuconfig', with prompt "Logging", which appears in the top menu. LOG
and its dependent symbols make up all of the logging Kconfig symbols
now.

Piggyback some minor cleanup.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>