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>
ARM: bcm2835: config.mk isn't needed

The entries in config.mk were needed so that U-Boot could be built
with an old version of the Raspberry Pi Foundation's toolchain. Without
them, the build would error out with:

...-ld: error: .../libgcc.a(_bswapsi2.o) uses VFP register arguments,
u-boot does not

However, none of the 3 toolchains in the latest version of their
tools.git, nor the Ubuntu/Linaro ARM compilers in at least Ubuntu Quantal
or Saucy, need these options set in order to compile a working U-Boot.
Hence, remove the options for simplicity.

Reported-by: Tom Rini <trini@ti.com>
Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>