ARM: rpi_b: set $fdtfile in default environment

U-Boot names the Raspberry Pi board rpi_b. This means that the common
expression for DTB filename ${soc}-${board}.dtb expands to
bcm2835-rpi_b.dtb. However, the DTB generated by the Linux kernel is
bcm2835-rpi-b.dtb. Set $fdtfile in U-Boot's environment so that scripts
look for the correct DTB filename.

An alternative would be to rename the U-Boot board to rpi-b. However,
that change would be far more invasive, and end up affecting users (i.e
they'd have to change their U-Boot build commands).

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>