add mruby module

Add mruby as an app.
When you want to add extensions(called mrbgems) on mruby, you'll need to specify git URI on build_config.rb, so this patch already includes some external mrbgems which tested on OSv.
Default cmdline argument will executes EShell, which is very simple shell for OSv/mruby environment.

Signed-off-by: Takuya ASADA <syuu@cloudius-systems.com>
Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>