Allow build wrappers

Closes #340

Adds two optional environment variables that, if specified, will be
used in lieu of the `build` command and its parameters during build.

1. `EDK_BUILD_CMD` - If present, the absolute path to an application
   to use for the edk build process that will be invoked instead of
   `build`. This is primarily used to allow a tool to wrap around
   `build` such that it will eventually call `build` internally.

2. `EDK_BUILD_PARAMS` - If present, these parameters will be passed
   to the build command. This is primarily used to pair wrapper-
   specific parameters with the wrapper passed in `EDK_BUILD_CMD`.

From a practical standpoint, these are the minimum changes needed
in the build tools to allow the CodeQL CLI to be invoked as a build
wrapper.

Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>

docs/usability/environment_variables.md

@@ -27,4 +27,14 @@ Web dependencies. After the initial fetch, this can greatly improve the performa
 calling `stuart_update` and make it far easier to switch between multiple branches or
 scopes where dependencies may change.
 
+## EDK_BUILD_CMD
+
+If present, the absolute path to an application to use for the edk build process will be
+invoked instead of `build`. This is primarily used to allow a tool to wrap around `build`.
+
+## EDK_BUILD_PARAMS
+
+If present, these parameters will be passed to the build command. This is primarily used to
+pair wrapper-specific parameters with the wrapper passed in `EDK_BUILD_CMD`.
+
 For more info, see [the External Dependencies docs](using_extdep.md).

edk2toolext/environment/uefi_build.py

@@ -310,7 +310,18 @@ def Build(self):
         pre_build_env_chk = env.checkpoint()
         env.set_shell_var('PYTHONHASHSEED', '0')
         env.log_environment()
-        ret = RunCmd("build", params)
+
+        edk2_build_cmd = self.env.GetValue("EDK_BUILD_CMD")
+        if edk2_build_cmd is None:
+            edk2_build_cmd = "build"
+        logging.debug("The edk2 build command is %s" % edk2_build_cmd)
+
+        edk2_build_params = self.env.GetValue("EDK_BUILD_PARAMS")
+        if edk2_build_params is None:
+            edk2_build_params = params
+        logging.debug("Edk2 build parameters are %s" % edk2_build_params)
+
+        ret = RunCmd(edk2_build_cmd, edk2_build_params)
         # WORKAROUND - Undo the workaround.
         env.restore_checkpoint(pre_build_env_chk)