configs: am335x_evm: Support distro bootcmds

configs: am335x_evm: Support distro bootcmds

Add support for distro bootcmds and network booting while retaining
backwards compatibility with the current "legacy" setup. With these
changes the default boot sequence becomes:

* SD card (standard distro boot)
* SD card (legacy boot)
* EMMC (standard distro boot)
* EMMC (legacy boot)
* Nand (legacy boot)
* PXE (standard distro boot)
* DHCP (standard distro boot)

The older boot scripts have some overlap with what the distro
bootcommands to however i've left them unchanged to prevent introduction
of subtle bugs.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>