PX4 Build: install-prereqs-ubuntu fixes

Current install-prereqs-ubuntu.sh clone PX4Firmware from PX4 repository which not compatible with APM.

This fix, 
1. Changes repo to diydrones/PX4Firmwares
2. Removes the need to edit config.mk
3. Install additional 'zip' package and 'ia32-libs' for 64bit Ubuntu.  

P.S. Script tested only on my Ubuntu 12 64Bit