Skip encrypted bootloader with no mbedtls (#522)

Author: will-v-pi

bootloaders/CMakeLists.txt

@@ -1 +1,6 @@
-add_subdirectory_exclude_platforms(encrypted host rp2040 rp2350-riscv)
+if (TARGET pico_mbedtls)
+    add_subdirectory_exclude_platforms(encrypted host rp2040 rp2350-riscv)
+else()
+    # Assume picotool has no signing support, if no pico_mbedtls available
+    message("Skipping encrypted bootloader example as pico_mbedtls unavailable")
+endif ()