fastboot: Rename fb_set_reboot_flag to fastboot_set_reboot_flag

Rename fb_set_reboot_flag to fastboot_set_reboot_flag so it matches
all other fastboot code in the global name space. Fix the guards around
them so that they're dependent on FASTBOOT, not just USB_FUNCTION_FASTBOOT.

Move the weak implementation of fastboot_set_reboot_flag to fb_common.c
so we can call it from non-USB fastboot code.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/board/amazon/kc1/kc1.c b/board/amazon/kc1/kc1.c
index d9ca183..031fd11 100644
--- a/board/amazon/kc1/kc1.c
+++ b/board/amazon/kc1/kc1.c
@@ -161,7 +161,7 @@
 	omap_die_id_get_board_serial(serialnr);
 }
 
-int fb_set_reboot_flag(void)
+int fastboot_set_reboot_flag(void)
 {
 	return omap_reboot_mode_store("b");
 }
diff --git a/board/lg/sniper/sniper.c b/board/lg/sniper/sniper.c
index 34a7a11..a7de4c2 100644
--- a/board/lg/sniper/sniper.c
+++ b/board/lg/sniper/sniper.c
@@ -173,7 +173,7 @@
 	omap_reboot_mode_store(reboot_mode);
 }
 
-int fb_set_reboot_flag(void)
+int fastboot_set_reboot_flag(void)
 {
 	return omap_reboot_mode_store("b");
 }