arm_boot: support board IDs more than 16 bits wide

Support passing a board ID value to the kernel in r1
that is more than 16 bits wide. This is needed to pass
the '-1 == invalid' value for boards which only support
device tree booting.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Mark Langsdorf <mark.langsdorf@calxeda.com>
