Roboleaf product configuration runner

The application rbcrun executes Starlark scripts that define Android product configurations.
See README.md for details.

Test: go test
Fixes: 180529448
Change-Id: I7d728b47d3f381b7052a0d7d51c9e698e5c2e316

tools/rbcrun/README.md

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+# Roboleaf configuration files interpreter
+
+Reads and executes Roboleaf product configuration files.
+
+## Usage
+
+`rbcrun` *options* *VAR=value*... [ *file* ]
+
+A Roboleaf configuration file is a Starlark script. Usually it is read from *file*. The option `-c` allows to provide a
+script directly on the command line. The option `-f` is there to allow the name of a file script to contain (`=`).
+(i.e., `my=file.rbc` sets `my` to `file.rbc`, `-f my=file.rbc` runs the script from `my=file.rbc`).
+
+### Options
+
+`-d` *dir*\
+Root directory for load("//path",...)
+
+`-c` *text*\
+Read script from *text*
+
+`--perf` *file*\
+Gather performance statistics and save it to *file*. Use \
+`       go tool prof -top`*file*\
+to show top CPU users
+
+`-f` *file*\
+File to run.
+
+## Extensions
+
+The runner allows Starlark scripts to use the following features that Bazel's Starlark interpreter does not support:
+
+### Load statement URI
+
+Starlark does not define the format of the load statement's first argument.
+The Roboleaf configuration interpreter supports the format that Bazel uses
+(`":file"` or `"//path:file"`). In addition, it allows the URI to end with
+`"|symbol"` which defines a single variable `symbol` with `None` value if a
+module does not exist. Thus,
+
+```
+load(":mymodule.rbc|init", mymodule_init="init")
+```
+
+will load the module `mymodule.rbc` and export a symbol `init` in it as
+`mymodule_init` if `mymodule.rbc` exists. If `mymodule.rbc` is missing,
+`mymodule_init` will be set to `None`
+
+### Predefined Symbols
+
+#### rblf_env
+
+A `struct` containing environment variables. E.g., `rblf_env.USER` is the username when running on Unix.
+
+#### rblf_cli
+
+A `struct` containing the variable set by the interpreter's command line. That is, running
+
+```
+rbcrun FOO=bar myfile.rbc
+```
+
+will have the value of `rblf_cli.FOO` be `"bar"`
+
+### Predefined Functions
+
+#### rblf_file_exists(*file*)
+
+Returns `True`  if *file* exists
+
+#### rblf_wildcard(*glob*, *top* = None)
+
+Expands *glob*. If *top* is supplied, expands "*top*/*glob*", then removes
+"*top*/" prefix from the matching file names.
+
+#### rblf_regex(*pattern*, *text*)
+
+Returns *True* if *text* matches *pattern*.
+
+#### rblf_shell(*command*)
+
+Runs `sh -c "`*command*`"`, reads its output, converts all newlines into spaces, chops trailing newline returns this
+string. This is equivalent to Make's
+`shell` builtin function. *This function will be eventually removed*.