Make the low available memory message a warning

Write the low available memory message to err rather than out so that it
appears more like a warning and is more noticeable.
Fix EXTERNAL_NUM_INTERRUPTS for atmega128rfa1 and atmega256rfr2
added the MailboxReadMessage example for the Yun
Merge pull request #1775 from ribbons/memory-usage

Improve low free memory reporting
Merge pull request #2044 from matthijskooijman/ide-1.5.x-interruptcount

Fix EXTERNAL_NUM_INTERRUPTS for atmega128rfa1 and atmega256rfr2
Merge pull request #2045 from Fede85/ide-1.5.x

added the MailboxReadMessage example for the Yun
Merge branch 'ide-1.5.x' into ide-1.5.x-avr-toolchain-gcc-4.8.1