Add Crystal Frequency menu item for Generic module

The Generic ESP8266 Module provides the basis to compile for anything not on the current list.  Although most current boards use the 26MHz crystal, there are many using the 40MHz.  This change permits these to be readily accommodated as generic ESP8266 boards.

Author: jwrw

boards.txt

@@ -38,6 +38,10 @@ generic.menu.CpuFrequency.80.build.f_cpu=80000000L
 generic.menu.CpuFrequency.160=160 MHz
 generic.menu.CpuFrequency.160.build.f_cpu=160000000L
 
+generic.menu.CrystalFreq.26=26 MHz
+generic.menu.CrystalFreq.40=40 MHz
+generic.menu.CrystalFreq.40.build.extra_flags=-DF_CRYSTAL=40000000
+
 generic.menu.FlashFreq.40=40MHz
 generic.menu.FlashFreq.40.build.flash_freq=40
 generic.menu.FlashFreq.80=80MHz