Fix regex for "make show_boards" on 1.5

In 1.5.6 the Due board has an underscore, this regex tweak allows for:

arduino_due_x_dbg  Arduino Due (Programming Port)
arduino_due_x      Arduino Due (Native USB Port)
Merge pull request #241 from sej7278/master

Fix regex for "make show_boards" on 1.5
Small typo, can lead to copy and paste fails

Signed-off-by: Joseivaldo Benito Junior <jrbenito@benito.qsl.br>
Merge pull request #247 from jrbenito/master

Fix a typo in README
Merge branch 'master' into auto-lib