SAMD21: Add build framework for DMA support.  Nothing there yet except for skeletal logic taken from SAM3/4.

SAML21: Since SERCOM5 usese a different output channel, it will also need a different GCLK generator

Lincoln60: Add a thttpd-binfs configuration.  That configuration still does not work on initial commit

Add simulator configuration for testing the Union Fs.  Fix a few bugs.  Almost works but still too flaky

Add support for a union file system that can be used to overlay and merge the content of two mounted file systems.