Create a build error if no bootloader is specified. (#16181)

* Create a build error if no bootloader is specified.

* Update builddefs/bootloader.mk

Co-authored-by: Ryan <fauxpark@gmail.com>

Co-authored-by: Ryan <fauxpark@gmail.com>

platforms/chibios/bootloaders/custom.c

@@ -16,8 +16,6 @@
 
 #include "bootloader.h"
 
-#pragma message "Unknown bootloader set, you may not be able to enter bootloader using software reset"
-
 __attribute__((weak)) void bootloader_jump(void) {}
 
 __attribute__((weak)) void enter_bootloader_mode_if_requested(void) {}