Make more clear that we are using private key for signing and public key for encryption

Author: pennam

boards.txt

@@ -82,8 +82,8 @@ envie_m7.build.alignment=32
 envie_m7.build.version=1.2.3+4
 envie_m7.menu.security.sien.build.postbuild.cmd="{tools.imgtool.path}/{tools.imgtool.cmd}" {tools.imgtool.flags}
 envie_m7.menu.security.sien.build.keys.keychain={runtime.platform.path}/libraries/MCUboot/default_keys
-envie_m7.menu.security.sien.build.keys.sign_key=ecdsa-p256-signing-key.pem
-envie_m7.menu.security.sien.build.keys.encrypt_key=ecdsa-p256-encrypt-key.pem
+envie_m7.menu.security.sien.build.keys.sign_key=ecdsa-p256-signing-priv-key.pem
+envie_m7.menu.security.sien.build.keys.encrypt_key=ecdsa-p256-encrypt-pub-key.pem
 envie_m7.menu.security.none.build.postbuild.cmd="{tools.imgtool.path}/{tools.imgtool.cmd}" exit
 envie_m7.compiler.mbed.arch.define=
 envie_m7.compiler.mbed.defines={build.variant.path}/defines.txt