Skip encrypted bootloader with no mbedtls

[will-v-pi]

Currently, a pico-examples build without the mbedtls submodule checked out in the SDK will fail, as picotool throws

ERROR: Cannot sign/hash partition table with no mbedtls

when making enc_bootloader, due to being compiled without mbedtls

Solve this by skipping enc_bootloader when there is no pico_mbedtls

[kilograham]

small nit, but the convention is to put the guard in the individual example's CMakeLists.txt unless it affects multiple examples

[lurch]

This example now gets correctly skipped inside a Docker image where I've cloned pico-sdk but not init-ed its submodules

[will-v-pi]

small nit, but the convention is to put the guard in the individual example's CMakeLists.txt unless it affects multiple examples

This will probably end up applying to multiple bootloader examples in the long run, so I think it's ok? It applies to anything that requires signing/hashing with picotool