novena-spl: Add support for booting from external SD

The factory image will be built using the external SD card, so it is
important that U-Boot supports starting from the external SD card.

Signed-off-by: Sean Cross <xobs@kosagi.com>
novena: Support booting off of the external SD card

Examine the bootsrc and determine how the system was booted.  Pull the
kernel and device tree file off of that boot medium.

Support for SATA has been started as well, but SPL does not yet support it.

Signed-off-by: Sean Cross <xobs@kosagi.com>
debian: Add changelog for r8

Signed-off-by: Sean Cross <xobs@kosagi.com>