revise bottom silkscreen to include useful documentation

[securelyfitz]

Back of the exodus intel board lists all pin mappings, openocd config, and flashrom commands.

Not much space, and not sure what is best to use it for, but some documentation might be worthwhile:

• mode switch meanings
• vtarget selection info
• vref/vtarget isolation
• flashrom command
• pin assignments
• bitmagic/LA header info