u-boot: further simplify the configuration

Remove all the bootsource selection mechanism and the horribly
complicated BR2_TARGET_UBOOT_DEFAULT_ENV thing, which wanted to be
generic, but was in fact very AT91-specific.

Just keep things simple: we build U-Boot with the board configuration
file specified in BR2_TARGET_UBOOT_BOARDNAME.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>