Stop passing around bootmem_base value.

For the calls to boot_relocate_fdt(), boot_get_cmdline(), and
boot_get_kbd(), the value of bootmem_base is always obtained by
calling getenv_bootm_low().  Since the value always comes from the
same source, the calling signature for those functions can be
simplified by making them call getenv_bootm_low() directly.

Signed-off-by: Grant Likely <grant.likely@linaro.org>