Add new env2cross command.

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+## Experimental command to convert environments to cross files
+
+Meson has a new command `env2mfile` that can be used to convert
+"environment variable based" cross and native compilation environments
+to Meson machine files. This is especially convenient for e.g. distro
+packagers so they can easily generate unambiguous configuration files
+for packge building.
+
+As an example here's how you would generate a cross file that takes
+its settings from the `CC`, `CXX`, `CFLAGS` etc environment variables.
+
+    meson env2mfile --cross --system=baremetal --cpu=armv7 --cpu-family=arm -o armcross.txt
+
+The command also has support for generating Debian build files using
+system introspection:
+
+    meson env2mfile --cross --debarch armhf -o debarmhf_cross.txt
+
+Note how you don't need to specify any system details, the command
+gets them transparently via `dpkg-architecture`.
+
+Creating a native file is done in the same way:
+
+    meson env2mfile --native -o current_system.txt
+
+This system will detect if the `_FOR_BUILD` environment variables are
+enabled and then uses them as needed.
+
+With this you should be able to convert any envvar-based cross build
+setup to cross and native files and then use those. Thit means, among
+other things, that you can then run your compilations from any shell,
+not just the special one that has all the environment variables set.
+
+As this functionality is still a bit in flux, the specific behaviour
+and command line arguments to use are subject to change. Because of
+this the main documentation has not yet been updated.
+
+Please try this for your use cases and report to us if it is working.
+Patches to make the autodetection work on other distros and platforms
+are also welcome.