portenta-h7: Fix incorrect board identifier

Author: sebromero

boards.txt

@@ -253,7 +253,7 @@ portentah7.build.float-abi=-mfloat-abi=softfp
 portentah7.build.extra_flags=
 portentah7.build.postbuild.cmd="{tools.imgtool.path}/{tools.imgtool.cmd}" exit
 portentah7.build.architecture=cortex-m7
-portentah7.build.board=PORTENTA_H7
+portentah7.build.board=PORTENTA_H7_M7
 portentah7.vid.0=0x2341
 portentah7.pid.0=0x005b
 portentah7.upload_port.0.vid=0x2341