ARM: bL_switcher: add kernel cmdline param to disable the switcher on boot

By adding no_bL_switcher to the kernel cmdline string, the switcher
won't be activated automatically at boot time.  It is still possible
to activate it later with:

	echo 1 > /sys/kernel/bL_switcher/active

Signed-off-by: Nicolas Pitre <nico@linaro.org>