execl():  Fix bad logic in counting the number of arguments.  This could never have worked.  Noted by Pierre-noel Bouteville

Convert all STM32F4-Discovery configurations (with base board) to configure the Ethernet driver to use the work queue (vs. interrupt level processing).

Add logic so that STM32 Ethernet drivier can avoid interrupt level processing and, instead, execute on the work thread.

SYSLOG: Add an option to use the syslog'ing device as the system consolution.  This option enables a low-level, write-only console device at /dev/console (similar to the low-level UART console device).  From Pierre-noel Bouteville.

STM32 SPI:  The source clock for SPI 4,5, and 6 should be PCLK2, not PCLK1 (for F411, F427, and F429).  Per David Sidrane.

IPv6 fixes:  Both ICMPv6 autonomous auto-configuration and the ICMPv6 router function appear to be fully functional.

STM32: Add an IPv6 configuration for the STM32F4-Discovery board (witht he STM32-DISCO_BB base board).  Verify that the STM32 Ethernet driver works with IPv6.