fastboot: Implement generic fastboot_set_reboot_flag

It is possible to implement fastboot_set_reboot_flag in a generic way
if BCB commands are turned on for a target. Using
bcb_set_reboot_reason allows to do this by simply passing string with
correct reboot reason that should be handled during next boot process.

If BCB are turned off, then bcb_set_reboot_reason would simply return
error, so it won't introduce any new behaviour for such targets.

Signed-off-by: Roman Kovalivskyi <roman.kovalivskyi@globallogic.com>
diff --git a/drivers/fastboot/fb_common.c b/drivers/fastboot/fb_common.c
index 736ce1c..cbcc368 100644
--- a/drivers/fastboot/fb_common.c
+++ b/drivers/fastboot/fb_common.c
@@ -10,6 +10,7 @@
  * Rob Herring <robh@kernel.org>
  */
 
+#include <bcb.h>
 #include <common.h>
 #include <command.h>
 #include <env.h>
@@ -90,7 +91,20 @@
  */
 int __weak fastboot_set_reboot_flag(enum fastboot_reboot_reason reason)
 {
-	return -ENOSYS;
+#if CONFIG_IS_ENABLED(FASTBOOT_FLASH_MMC_DEV)
+	static const char * const boot_cmds[] = {
+		[FASTBOOT_REBOOT_REASON_BOOTLOADER] = "bootonce-bootloader",
+		[FASTBOOT_REBOOT_REASON_FASTBOOTD] = "boot-fastboot",
+		[FASTBOOT_REBOOT_REASON_RECOVERY] = "boot-recovery"
+	};
+
+	if (reason >= FASTBOOT_REBOOT_REASONS_COUNT)
+		return -EINVAL;
+
+	return bcb_write_reboot_reason(CONFIG_FASTBOOT_FLASH_MMC_DEV, "misc", boot_cmds[reason]);
+#else
+    return -EINVAL;
+#endif
 }
 
 /**