build_library: Fix pkg_use_enabled

"equery uses" ignores forced or masked USE flags by default. In our
case, the selinux USE flag is forced, so stop ignoring it with
--forced-masked flag. Update the regexp to catch the forced USE flags
too and modernize the function a bit.

Signed-off-by: Krzesimir Nowak <knowak@microsoft.com>

build_library/board_options.sh

@@ -16,13 +16,19 @@ ARCH=$(get_board_arch ${BOARD})
 
 # check if any of the given use flags are enabled for a pkg
 pkg_use_enabled() {
-  local pkg="$1"
-  shift
-  # for every flag argument, turn it into `-e ^+flag` for grep
-  local grep_args="${@/#/-e ^+}"
+  local pkg="${1}"; shift
 
-  equery-"${BOARD}" -q uses "${pkg}" | grep -q ${grep_args}
-  return $?
+  # for every flag argument, turn it into a regexp that matches it as
+  # either '+${flag}' or '(+${flag})'
+  local -a grep_args=()
+  local flag
+  for flag; do
+      grep_args+=( -e '^(\?+'"${flag}"')\?$' )
+  done
+  local -i rv=0
+
+  equery-"${BOARD}" --quiet uses --forced-masked "${pkg}" | grep --quiet "${grep_args[@]}" || rv=$?
+  return ${rv}
 }
 
 # Usage: pkg_version [installed|binary|ebuild] some-pkg/name