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

@@ -55,7 +55,7 @@ case "$1" in
 	printf "Prefix out of range (%s)\n" "$prefix" >&2
 	exit 1
     fi
-    netmask=$(((0xffffffff << (32 - prefix)) & 0xffffffff))
+    prefix2netmask netmask "$prefix" || exit 1
     shift
     ;;
 *)

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

@@ -144,3 +144,16 @@ ip2str() {
     export -- "$__var=$((__n >> 24)).$(((__n >> 16) & 255)).$(((__n >> 8) & 255)).$((__n & 255))"
 }
 
+# convert prefix into an integer bitmask
+prefix2netmask() {
+    local __var="$1" __n="$2"
+    assert_uint32 "$__n" || return 1
+
+    if [ "$__n" -gt 32 ]; then
+	printf "Prefix out-of-range (%s)" "$__n" >&2
+	return 1
+    fi
+
+    export -- "$__var=$(((~(uint_max >> __n)) & uint_max))"
+}
+