change tools.imgtool.build.pattern to tools.imgtool.flags

The property had the same form as the special `tools.TOOL_NAME.ACTION.pattern` properties
However, there is not a `build` action, the form of the property gives the impression that it is one that has special treatment by the build system.

It looks like the convention is `*.flags`

Author: umbynos

boards.txt

@@ -80,7 +80,7 @@ envie_m7.build.slot_size=0x1E0000
 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.build.pattern}
+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.sign_key=ecdsa-p256-signing-key.pem
 envie_m7.menu.security.sien.build.keys.encrypt_key=ecdsa-p256-encrypt-key.pem

platform.txt

@@ -193,7 +193,7 @@ tools.picotool.upload.pattern="{path}/{cmd}" {upload.verbose} -D "{build.path}/{
 
 tools.imgtool.path={runtime.tools.imgtool.path}
 tools.imgtool.cmd=imgtool
-tools.imgtool.build.pattern=sign --key "{build.keys.keychain}/{build.keys.sign_key}" --encrypt "{build.keys.keychain}/{build.keys.encrypt_key}" "{build.path}/{build.project_name}.bin" "{build.path}/{build.project_name}.bin" --align {build.alignment} --max-align {build.alignment} --version {build.version} --header-size {build.header_size} --pad-header --slot-size {build.slot_size}
+tools.imgtool.flags=sign --key "{build.keys.keychain}/{build.keys.sign_key}" --encrypt "{build.keys.keychain}/{build.keys.encrypt_key}" "{build.path}/{build.project_name}.bin" "{build.path}/{build.project_name}.bin" --align {build.alignment} --max-align {build.alignment} --version {build.version} --header-size {build.header_size} --pad-header --slot-size {build.slot_size}
 
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