Add documentation on Musca-A1 signing key

Author: Michael Schwarcz

tools/targets/musca_a1-root-rsa-2048.md

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+# Musca-A1 RSA keypair
+
+A default RSA key pair is given to the Musca-A1 target.
+
+Public key was pre-compiled to `targets/TARGET_ARM_SSG/TARGET_MUSCA_A1/TARGET_MUSCA_A1_NS/device/mcuboot.bin` and private key is in `musca_a1-root-rsa-2048.pem`.
+
+DO NOT use them in production code, they are exclusively for testing!
+
+Private key must be stored in a safe place outside of the repository.
+
+`tools/psa/tfm/bin_utils/imgtool.py` can be used to generate new key pairs.