Tiva: Add build framework and skeleton files for Tiva I2C driver.  Initial commit is just the STM32 I2C driver with name changes and STM32-specific logic removed

Traveler:  Add logic to limit the frame rate.  This is kind of a silly feature -- why would you ever want to limit the frame rate?  Well, you need to that on the simulated platform to make bandwidth for other things to run like the simulated timer

stdint.h:  Don't use hex values to specify minimum values of fixed width, signed values.  Hex values are inherently unsigned and not usable for this purpose in all contexts

avsprintf():  Fix a bug in usage of va_list on x86.  On x86, va_list is a pointer to a single copy on the stack.  avsprintf() calls lib_vsprintf() twice and so traverses the va_list twice using va_start.  va_start modifies that single copy on the stack so that the second call to lib_vsprintf() fails.  This appears to be an issue with x86 only so far

Set the GPIO_SPEED_50MHz on all F2 and F4 SPI pin configurations.  This is based on an F411 SPI1 errata but the fixed is generalized to all SPI and all F2 and F4 (let me know if this introduces any other issues).  Discovered and fixed by Sebastien Lorquet after much consternation.

Add sys/custom_file.h.  Used when CUSTOM_FILE_IO is define and avoids re-definition errors about the FILE define.  From Thomas Gruber via the PX4 repository

In message queue created return ENOSPC error if size exceeds the configured size of pre-allocatd messages; Use ENOSPC vs ENOMEM per OpenGroup.org.  From  Pierre-Noel Bouteville