[autotest] Support scheduling suite jobs need firmware update in suite_scheduler.

Sample entry in suite_scheduler.ini will be like:

This should create faft_ec suite when new build created on firmware branch,
tot CrOS build is used. Test is from ChromeOS branch.
Test command:
/usr/local/autotest/site_utils/suite_scheduler/suite_scheduler.py -b \
 -d /usr/local/autotest/logs -f /usr/local/autotest/test_suite_scheduler.ini \
 -r /tmp/_autotmp_0Wmh0i_suite_scheduler --e new_build  \
 -i veyron_jerry-firmware/R41-6588.106.0

[faft]
run_on: new_build
suite: faft_ec
branch_specs: firmware
cros_build_spec: tot
test_source: cros
boards: veyron_jerry
pool: faft_test

This should create faft_ec suite when new build created on R44 branch,
tot firmware build on firmware branch is used. Test is from firmware branch.
Test command:
/usr/local/autotest/site_utils/suite_scheduler/suite_scheduler.py -b \
 -d /usr/local/autotest/logs -f /usr/local/autotest/test_suite_scheduler.ini \
 -r /tmp/_autotmp_0Wmh0i_suite_scheduler --e new_build  \
 -i veyron_jerry-release/R44-7077.90.0

[faft2]
run_on: new_build
suite: faft_ec
branch_specs: ==tot-2
firmware_rw_build_spec: firmware
test_source: firmware
boards: veyron_jerry
pool: faft_test

BUG=chromium:513409
TEST=local run suite_scheduler command as mentioned in commit message.

Change-Id: I8976c5af6ff771cd0ca409e55ffbbe1634c4cf4d
Reviewed-on: https://chromium-review.googlesource.com/288900
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Wai-Hong Tam <waihong@chromium.org>
Reviewed-by: Simran Basi <sbasi@chromium.org>
Reviewed-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
diff --git a/site_utils/suite_scheduler/constants.py b/site_utils/suite_scheduler/constants.py
index 8f90394..aec9ab2 100644
--- a/site_utils/suite_scheduler/constants.py
+++ b/site_utils/suite_scheduler/constants.py
@@ -13,3 +13,14 @@
     @var POOL_PREFIX The stright with which pool labels are prefixed."""
     BOARD_PREFIX = 'board:'
     POOL_PREFIX = 'pool:'
+
+
+class Builds:
+    """Constants related to build type.
+
+    @var FIRMWARE_RW: The string indicating the given build is used to update
+                      RW firmware.
+    @var CROS: The string indicating the given build is used to update ChromeOS.
+    """
+    FIRMWARE_RW = 'firmware_rw'
+    CROS = 'cros'
\ No newline at end of file