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: e306779 (rockchip: make boot_mode related codes reused across all platforms)
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>

arch/arm/mach-rockchip/Makefile

@@ -23,9 +23,11 @@ obj-spl-$(CONFIG_ROCKCHIP_RK3399) += rk3399-board-spl.o spl-boot-order.o
 
 ifeq ($(CONFIG_SPL_BUILD)$(CONFIG_TPL_BUILD),)
 
-ifneq ($(CONFIG_ROCKCHIP_BOOT_MODE_REG),0)
+# Always include boot_mode.o, as we bypass it (i.e. turn it off)
+# inside of boot_mode.c when CONFIG_BOOT_MODE_REG is 0.  This way,
+# we can have the preprocessor correctly recognise both 0x0 and 0
+# meaning "turn it off".
 obj-y += boot_mode.o
-endif
 
 obj-$(CONFIG_ROCKCHIP_RK3188) += rk3188-board.o
 obj-$(CONFIG_ROCKCHIP_RK322X) += rk322x-board.o

arch/arm/mach-rockchip/boot_mode.c

@@ -9,6 +9,15 @@
 #include <asm/io.h>
 #include <asm/arch/boot_mode.h>
 
+#if (CONFIG_ROCKCHIP_BOOT_MODE_REG == 0)
+
+int setup_boot_mode(void)
+{
+	return 0;
+}
+
+#else
+
 void set_back_to_bootrom_dnl_flag(void)
 {
 	writel(BOOT_BROM_DOWNLOAD, CONFIG_ROCKCHIP_BOOT_MODE_REG);
@@ -74,3 +83,5 @@ int setup_boot_mode(void)
 
 	return 0;
 }
+
+#endif