#3 Add new avr devices support

[rr]

Some new devices are not supported by avarice, like the atmega324 variants: atmega324a and atmega324pa (only atmega324p is supported).
I've tested with version 2.10.
I'm using the ice-insight scripts, and avarice detects an unknown part signature and is not able to proceed.
Please check all other devices and different signatures that are not supported

[Joerg Wunsch]

Please make a reasonable attempt to add new devices yourself you
are in dire need for. Usually, this could easily be done by cloning
a respective entry in src/devdescr.cc.

Note that due to the way the device auto-detection works, only
devices with a different device ID need an entry of their own. This
singles out most of the "A" derivatives, as they are using the same
device ID as their non-A counterparts.

Using the -P option to override the device ID is merely meant to be
an option to circumvent the device autodetection, rather than the
normal operation.