Move bootloader main to examples

This should remove some confusion around embassy-boot-* being a library
vs. a binary. The binary is now an example bootloader instead.

examples/boot/application/stm32wl/.cargo/config.toml

@@ -0,0 +1,9 @@
+[target.'cfg(all(target_arch = "arm", target_os = "none"))']
+# replace your chip as listed in `probe-run --list-chips`
+runner = "probe-run --chip STM32WLE5JCIx"
+
+[build]
+target = "thumbv7em-none-eabihf"
+
+[env]
+DEFMT_LOG = "trace"