davinci: spl: use bootcfg to select boot device

davinci: spl: use bootcfg to select boot device

Right now the SPL is trying to load u-boot based on defines, i.e. one
has to define CONFIG_SPL_NAND_SIMPLE to boot from NAND,
or CONFIG_SPL_SPI_LOAD to boot from SPI FLASH, etc...
This prevent us from having a single SPL image that is able to boot from
all media, and one need to build an image for each medium. This
commit is replacing the #ifdef that select the boot medium by reading
the value of the boot pins (via the BOOTCFG register).

Now a single SPL image will be able to read from the boot pin to know
which device should be used to load u-boot.

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Reviewed-by: Tom Rini <trini@konsulko.com>