sanitycheck: default to assertions being enabled

Changed the default behavior for when we run sanitycheck to match the -R
flag that turns on assertions.  Introduced a --disable-asserts option if
we desire to explicitly turn of asserts.

This matches behavior that our CI builds have been doing and addresses
part of #5726.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
diff --git a/scripts/sanitycheck b/scripts/sanitycheck
index f0212e1..87aab37 100755
--- a/scripts/sanitycheck
+++ b/scripts/sanitycheck
@@ -2206,7 +2206,12 @@
         help="Execute time-consuming test cases that have been marked "
         "as 'slow' in testcase.yaml. Normally these are only built.")
     parser.add_argument("-R", "--enable-asserts", action="store_true",
-                        help="Build all test cases with assertions enabled.")
+                        default=True,
+                        help="Build all test cases with assertions enabled. "
+                        "Default to assertions being enabled.")
+    parser.add_argument("--disable-asserts", action="store_false",
+                        dest="enable_asserts",
+                        help="Build all test cases with assertions disabled.")
     parser.add_argument("-Q", "--error-on-deprecations", action="store_false",
                         help="Error on deprecation warnings.")