Enable TM4C1294-launchpad board build

drivers/spi:  Add an SPI character driver that will permit access to the SPI bus for testing purposes.  This driver is a simple wrapper around spi_transfer().

Add tiva PWM lower-half driver implementation

Extend I/O Expander interrupt controls; Add test of level sensitve interrupt to Simulated I/O expander

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.

FAT performance improvement. In large files, seeking to a position fromt he beginning of the file can be very time consuming.  ftell does lssek(fd, 0, SET_CURR).  In that case, that is wasted time since we are going to seek to the same position.  This fix short-circutes fat_seek() in all cases where we attempt to seek to curren position.  Suggested by Nate Weibley

I/O Expanders:  Interrupt detection logic should not ignore the no-change case.  Still need to handle level interrupts even with no change.

Fix various issues with I/O expander and GPIO lower half drivers from testing with simulated I/O expander

SIM I/O Expander: Add logic to simulate inverted pins and to generate toggle values on interrupt input pins

Back out last Makefile.unix change.  After puzzling about this for a while, I moved the definitions to apps/Make.defs.  Also includes some cosmetic changes to GPIO lower half driver comments.