Can put 1R--10R resistor in commoned tail of NPNs to limit surge current
and to enable current monitoring (eg for stall) by AVR.

Can use 2N2222A and complement.

Can use Zetex ZTX1049A/ZTX968 as per http://www.robotroom.com/BipolarHBridge4.html 

Should split supply from AVR to avoid sags with motor running,
star wired back to supply, poss with ferrite bead in motor supply
and/or RC (plus poss diode in parallel with R) to keep AVR going
through sags and even while batteries are being changed. 

Or supply to AVR through Schottky and to small supercap can keep it running
whatever else happens to maintain RTC eg during battery swap out.
Has to avoid doing anything energetic when (say) cap voltage sags below
2V to give some headroom.  Might want to power RFM22 (etc) from raw
supply instead at risk of having to reset it, or just don't run it unless
supply good and motor not running.