rockchip: fix turning off boot-mode via Kconfig

The ROCKCHIP_BOOT_MODE_REG option defaults to a hex value, so 0 will
show as 0x0 if a default is provided and changed via Kconfig.
However, it still will show as 0, if no default is given.

Consequently, the "is set to something other than 0" test in a
Makefile is cumbersome.  Instead this check can easily be performed in
the C-code.

This removes the ifeq-check from mach-rockchip/Makefile, adds a
matching #if-check to boot_mode.c and fixes resulting link issues (if
boot_mode.o was not included due to the Makefile check) by defining a
stub function (in case the functionality is not built in) for
setup_boot_mode in boot_mode.c.

Fixes: e3067793 (rockchip: make boot_mode related codes reused across all platforms)
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>