core+flash: Added optional serial number

Some AVR have a serial number readable via LPM.

Changed avr_flash to support the SIGRD bit, and added a LPM IOCTL that
handles both that and the BLTSET bit to read fuses and lock bits

Also added a test with a mega644p that has that signature

Signed-off-by: Michel Pollet <[email protected]>

Author: buserror