Add check of return value in drivers affected by last change:  Report the error on a failure to set the bit order.

STM32 and EFM32 SPI drivers adopted an incompatible conventions somewhere along the line.  The set the number of bits to negative when calling SPI_SETBITS which had the magical side-effect of setting LSB first order of bit transmission.  This is not only a hokey way to pass control information but is supported by no other SPI drivers.

This change three things:  (1) It adds HWFEAT_LSBFIRST as a new H/W feature.  (2) It changes the implementations of SPI_SETBITS in the STM32 and EFM32 derivers so that negated bit numbers are simply errors and it adds the SPI_HWFEATURES method that can set the LSB bit order, and (3) It changes all calls with negative number of bits from all drivers: The number of bits is now always positive and SPI_HWFEATUREs is called with HWFEAT_LSBFIRST to set the bit order.

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