Add configuration to prevent selection of Windows native toolchains when using Ubuntu under Windows 10

Back out most of 34be3e7c and update README again.  Windows native tools cannot be used with Ubuntu under Windows 10 now.  For Cygwin, that support depends on the 'cygpath -w' tool to convert POSIX paths to Windows paths.  There is no corresponding tool for Ubuntu under Windows 10.

Revert "sam_tc_clockselect() reworked to calculate frequency error using smallest possible divisor minimizing the frequency error rather than largest possible divisor which maximized the error."

This reverts commit 5d5851a5.

sam_tc_clockselect() reworked to calculate frequency error using smallest possible divisor minimizing the frequency error rather than largest possible divisor which maximized the error.

Add tools/sethost.sh.   This is a script that you can use to quickly change the host platform from Linux to Windows/Cygwin.  Might save you a lot of headaches.

Add support for the NXP Freedom-K64F board.  This is primarily the work of Jordan Macintyre.  I leveraged this code from https://github.com/jmacintyre/nuttx-k64f

*  This driver supports the 'I2C lcd backpack' design that is based on the PCF8574 io expander.  There's a myriad of different vendors of such, but they are principally the same, save wiring and minor features like jumpers for I2C addresses.  This driver supports known and unknown variants.
*  The interface board supports HD44780-based LCD modules up to 4x32, and this driver accommodates all those formats.