Android: allow host OS to be passed to envsetup.

build/android/envsetup_functions.sh

Index: build/android/envsetup_functions.sh
diff --git a/build/android/envsetup_functions.sh b/build/android/envsetup_functions.sh
index b6cff269becbd6a63a23bb6277ebe19fb3833fe2..e9519f550a9f72caaa373d9582231f9d3fbdd523 100755
--- a/build/android/envsetup_functions.sh
+++ b/build/android/envsetup_functions.sh
@@ -169,12 +169,16 @@ print_usage() {
 # --help          Prints out help message.
 ################################################################################
 process_options() {
+  host_os=$(uname -s | sed -e 's/Linux/linux/;s/Darwin/mac/')
   try_32bit_host_build=
   while [[ $1 ]]; do
     case "$1" in
       --target-arch=*)
         target_arch="$(echo "$1" | sed 's/^[^=]*=//')"
         ;;
+      --host-os=*)
+        host_os="$(echo "$1" | sed 's/^[^=]*=//')"
+        ;;
       --try-32bit-host)
         try_32bit_host_build=true
         ;;