sysroot_lib: Convert to class.

Convert the sysroot handling logic to a class to facilitate
encapsulation.

BUG=None.
TEST=Trybots.

CQ-DEPEND=CL:263767

Change-Id: Ie65701cb3ab125cd589512a6f8fba1b85f0cd447
Reviewed-on: https://chromium-review.googlesource.com/264305
Reviewed-by: Steve Fung <stevefung@chromium.org>
Trybot-Ready: Bertrand Simonnet <bsimonnet@chromium.org>
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Commit-Queue: Bertrand Simonnet <bsimonnet@chromium.org>
diff --git a/scripts/cros_generate_sysroot.py b/scripts/cros_generate_sysroot.py
index 83e4db0..36e9791 100644
--- a/scripts/cros_generate_sysroot.py
+++ b/scripts/cros_generate_sysroot.py
@@ -70,8 +70,8 @@
 
   def _InstallToolchain(self):
     # Create the sysroot's config.
-    config = sysroot_lib.GenerateBoardConfig(self.sysroot, self.options.board)
-    sysroot_lib.WriteSysrootConfig(self.sysroot, config)
+    sysroot = sysroot_lib.Sysroot(self.sysroot)
+    sysroot.WriteConfig(sysroot.GenerateBoardConfig(self.options.board))
     cros_build_lib.RunCommand(
         [os.path.join(constants.CROSUTILS_DIR, 'install_toolchain'),
          '--noconfigure', '--sysroot', self.sysroot])