drivers/wireless/nrf24l01.c:  Review last PR.  Also got enthused and did major re-work to file to bring it closer to the NuttX coding style.  Fixed a few compile time warnings.

- signal POLLIN if there is already data in the FIFO
- send ETIMEDOUT to userspace after 2 seconds if TX IRQ was not received
- handle FIFO overflow
- handle invalid pipes/empty FIFO
- multiple cosmetics (missing static, duplicate define, missing \n)

stm32_gpiosetevent: GPIO IRQ logic no longer returns the xcpt_t oldhandler.  This value is useless and dangerous after the recent changes to interrupt argument passing.

Provide a user defined callback context for irq's, such that when
registering a callback users can provide a pointer that will get
passed back when the isr is called.

drivers/:  Remove all explicit use of 'hidden' macro _info.  Code must never use this directly.  Code must always use a debug macro such as info which is basic on _info but can be appropriately filtered.

SPI bit order:  Add configuration setting to indicate if an architecture-specif SPI implementation does or does not support LSB bit order.

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.

Remove lowsyslog().  The new syslog() includes all of the functionality of lowsyslog().  No longer any need for two interfaces.

drivers/wireless:  Change some *err() message to *info() messages if what was a *dbg() message does not indicate and error condition.