Portenta M7: boards.txt: fix keychain runtime path

Author: pennam

boards.txt

@@ -81,7 +81,7 @@ envie_m7.build.header_size=0x20000
 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.hardware.path}/mbed/libraries/MCUboot/default_keys
+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.none.build.postbuild.cmd="{tools.imgtool.path}/{tools.imgtool.cmd}" exit