Fixup bootloaders. (#16256)

keyboards/jm60/jm60.c

@@ -20,3 +20,8 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 void board_init(void) {
 	AFIO->MAPR |= AFIO_MAPR_SWJ_CFG_JTAGDISABLE;
 }
+
+void bootloader_jump(void) {
+    // This board doesn't use the "standard" stm32duino bootloader. There's no information on how to jump to the custom bootloader, so all we can do here is reset.
+    NVIC_SystemReset();
+}

keyboards/jm60/rules.mk

@@ -4,6 +4,9 @@ MCU = STM32F103
 MCU_LDSCRIPT = jm60_bootloader
 BOARD = ST_NUCLEO64_F103RB
 
+# Bootloader selection
+BOOTLOADER = custom
+
 # Build Options
 #   change yes to no to disable
 #