Support PSA Crypto from TF-M

[LDong-Arm]

Use only the standard PSA Crypto API, to support TF-M:

• Check PSA Crypto API major version instead of Mbed TLS version, to avoid direct dependency on an Mbed TLS header.
• Call mbedtls_psa_crypto_free() (not part of the PSA Crypto API) when using Mbed OS PSA only.
• Enable SHA-256 hash for PKCS1v15 message signing, because CryptoCell 312 does not allow no hash.
• Reword: Mbed Crypto -> PSA Crypto
• Add ARM_MUSCA_S1 to Travis as a TF-M target. Also update the Ubuntu image to Focal, hoping to solve the bootloader image signing failure.

General fixes & improvements

• Fix Travis build
• Update README with Mbed CLI 2 instructions
• Enable PSA by default (to make it slightly easier for users to enable Mbed Crypto)

[LDong-Arm]

Now Travis passes and this PR is ready for review!