update overclock options
update overclock options

Due to some people reporting SD card corruption, back off on overclocking
core_freq for all overclock options other than turbo. For turbo, bump SDRAM
overclock to 600 and warn the user that some people have reported SD card
corruption.

senden9/raspi-config,jrobertson/raspi-config,asb/raspi-config,c0d3z3r0/raspi-config,marthinx/raspi-config,penn5/raspi-config