EFM32: Rename efm_spi_initialize to efm_spibus_initialize for compatibility with corresponding changes to other platforms

All SPI-based device drivers needs to call SPI_HWFEATURES() with zero in order to co-exist with drivers that use H/W features

This patch manages the packet buffer used by the upper layers by making sure it is always set to a valid transmit buffer that can be used by the MAC-NET core.  The only exception to this is when the upper layer re-uses a receive buffer to send a response. In this case, the updated receive buffer is swapped with an empty transmit buffer.  If there is no empty transmit buffer available, the packet will be dropped.

Signed-off-by: Andrew Webster <awebster@arcx.com>

Kinetis enet: add support for DBSWP

Signed-off-by: Andrew Webster <awebster@arcx.com>

Kinetis enet: leave the write buffer wrap bit alone.  The wrap buffer bit is initialized when the buffers are set up and should not be changed later

Kinetis enet: support multiple transmits between interrupts.  Accounts for the unlikely event that more than one packet has been transmitted before the tx done is processed

Kinetis enet: use rounded up MTU size for buffers. The buffers must be evenly divisible by 16, so round up the MTU size and use that for the buffer sizes