I am looking for a reference designs for beginners, who would like to
use open-source tools to develop some softwre using cross compiler on
host machine for those reference designs and should be able to
downloadble using serial port(embedded linux dev/GCC), basically a
beginner can buy this hardware and write some software and should be
downloadable onto target board. I am looking for in-expensive, which
should contain a processor, RAM, flash/rom, serial interfaces, LAN or
USB interface.

Please let me know where can I get it, appreciated. Thanks in advance.