envconfig: Add [constants] section in machine files

Machine files already supports `+` operator as an implementation detail,
since it's using eval(). Now make it an officially supported feature and
add a way to define constants that are used while evaluating an entry
value.

1 files changed, 20 insertions, 0 deletions

diff --git a/docs/markdown/snippets/machine_file_constants.md b/docs/markdown/snippets/machine_file_constants.md
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index 0000000..84b0848
--- /dev/null
+++ b/docs/markdown/snippets/machine_file_constants.md
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+## Machine file constants
+
+Native and cross files now support string and list concatenation using the `+`
+operator, and joining paths using the `/` operator.
+Entries defined in the `[constants]` section can be used in any other section.
+An entry defined in any other section can be used only within that same section and only
+after it has been defined.
+
+```ini
+[constants]
+toolchain = '/toolchain'
+common_flags = ['--sysroot=' + toolchain + '/sysroot']
+
+[properties]
+c_args = common_flags + ['-DSOMETHING']
+cpp_args = c_args + ['-DSOMETHING_ELSE']
+
+[binaries]
+c = toolchain + '/gcc'
+```