The SYS_BOOT-based fallback shouldn't only check for one of the conditions of
use and then let the switch/case handle each boot device without enforcing the
conditions for each type of boot device again.

For instance, this behaviour would trigger the fallback for UART when
BOOT_DEVICE_UART is defined, CONFIG_SPL_YMODEM_SUPPORT is enabled (which should
be a show-stopper) and e.g. BOOT_DEVICE_USB is enabled and not
CONFIG_SPL_USB_SUPPORT.
Separating the logic for USB and UART solves this.

In addition, this adds support for more peripheral devices (USBETH and CPGMAC)
to the fallback mechanism. Note that the USBETH boot device should always be
different from the USB boot device (each should match a different bootrom
handoff case).

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Acked-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Tested-by: Hannes Schmelzer <oe5hpm@oevsv.at>