Tiva Ethernet: Integrate use of workqueue so the network processing is not done at the interrupt level

Tiva Ethernet:  First cut at TM4C129X Ethernet driver.  Initial commit is basically just the STM32 Ethernet driver with modifications for a clean compilation in the Tiva environment

Signed-off-by: Marco Krahl <ocram.lhark@gmail.com>

Signed-off-by: Marco Krahl <ocram.lhark@gmail.com>

stm32: fix faulty access to non existing layer.  This disables operation that requires double layer support, when configured for single layer only.

Signed-off-by: Marco Krahl <ocram.lhark@gmail.com>

Tiva SSI: Fix oversight in last commit.  Would only fixe the case where the single SSI enabled was SSI0

Tiva SSI: Fix some recent breakage to the Tiva SSI driver for the case where only one SSI modules is enabled

Remove STM32-specific RX flow control logic from the upper level serial driver to the lower level STM32 serial driver

Tiva SSI and board configurations: hange negative Tiva logic CONFIG_SSIx_DISABLE to positive logic CONFIG_TIVA_SSIx.  Add support for SSI2 and SSI3