Adding Core Clock 16MHz/20MHz menu

Author: nekogumi

Diff for: boards.txt

@@ -86,4 +86,12 @@ nona4809.menu.mode.on.build.328emulation=-DAVR_NANO_4809_328MODE
 nona4809.menu.mode.off=None (ATMEGA4809)
 nona4809.menu.mode.off.build.328emulation=
 
+menu.freq=Core Frequency
+nona4809.menu.freq.on=20 MHz
+nona4809.menu.freq.on.build.f_cpu=20000000L
+nona4809.menu.freq.on.bootloader.OSCCFG=0x02
+nona4809.menu.freq.off=16 MHz
+nona4809.menu.freq.off.build.f_cpu=16000000L
+nona4809.menu.freq.off.bootloader.OSCCFG=0x01
+
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