base-files: ipcalc.sh: Add prefix-to-netmask conversion

Seems like it might be used in other places, so factor it into the
library.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>

package/base-files/files/bin/ipcalc.sh

@@ -44,7 +44,7 @@ case "$1" in
 	echo "Prefix out of range ($n)" >&2
 	exit 1
     fi
-    netmask=$((~((1 << (32 - n)) - 1) & 0xffffffff))
+    netmask=$(prefix2netmask "$n") || exit 1
     shift
     ;;
 *)

package/base-files/files/lib/functions/ipv4.sh

@@ -131,3 +131,12 @@ int2ip() {
     echo "$((n >> 24)).$(((n >> 16) & 255)).$(((n >> 8) & 255)).$((n & 255))"
 }
 
+# convert prefix into an integer bitmask
+prefix2netmask() {
+    local n="$(check_uint32 "$1")"
+
+    [ "$n" -gt 32 ] && __die "Prefix out-of-range ($n)"
+
+    echo "$(((~(uint_max >> n)) & uint_max))"
+}
+