Add initial support for tinyuf2 bootloader (when hosted on F411 blackpill) (#12600)

* Add support for jumping to tinyuf2 bootloader. Adds blackpill UF2 example.

* Update flashing.md

* Update chconf.h

* Update config.h

* Update halconf.h

* Update mcuconf.h

quantum/mcu_selection.mk

@@ -304,7 +304,12 @@ ifneq ($(findstring STM32F411, $(MCU)),)
   # Linker script to use
   # - it should exist either in <chibios>/os/common/ports/ARMCMx/compilers/GCC/ld/
   #   or <keyboard_dir>/ld/
-  MCU_LDSCRIPT ?= STM32F411xE
+  ifeq ($(strip $(BOOTLOADER)), tinyuf2)
+    MCU_LDSCRIPT ?= STM32F411xE_tinyuf2
+    FIRMWARE_FORMAT ?= uf2
+  else
+    MCU_LDSCRIPT ?= STM32F411xE
+  endif
 
   # Startup code to use
   #  - it should exist in <chibios>/os/common/startup/ARMCMx/compilers/GCC/mk/