flatcar_workon: Bring back the --host flag

Telling flatcar_workon to work on SDK instead of a board could be
implemented either by passing an empty value for --board flag and
modifying `${FLAGS_board+foo}` to `${FLAGS_board:+foo}` or by adding
the `--host` boolean flag and killing the value of `--board` flag when
`--host` is true.

I went with the latter as it clearly conveys meaning at the call
sites.

flatcar_workon

@@ -6,6 +6,8 @@
 
 DEFINE_string board "${DEFAULT_BOARD}" \
   "The board to set package keywords for."
+DEFINE_boolean host "${FLAGS_FALSE}" \
+  "Uses the host instead of board"
 
 FLAGS_HELP="usage: $0 <command> [flags] <atom>
 commands:
@@ -17,6 +19,13 @@ eval set -- "${FLAGS_ARGV}"
 
 set -euo pipefail
 
+# If both board and host are specified, just use host, because board
+# does not have to be specified and may come from default, in which
+# case there's no way to override.
+if [[ -n ${FLAGS_board} ]] && [[ ${FLAGS_host} = "${FLAGS_TRUE}" ]]; then
+  unset FLAGS_board # kill board
+fi
+
 # /etc/portage under either / or /build/<board>.
 ETC_PORTAGE=${FLAGS_board+/build/${FLAGS_board}}/etc/portage