fix(rules): Initialize SFD2UFOFLAGS with a default value

This patch provides the variable SFD2UFOFLAGS for the `make` machinery.

Author: jgmbenoit

rules/rules.mk

@@ -77,6 +77,7 @@ TTXFLAGS ?= -v
 WOFF2COMPRESSFLAGS ?=
 GFTOOLSFLAGS ?=
 PYTHONFLAGS ?= -d
+SFD2UFOFLAGS ?=
 SFDNORMALIZEFLAGS ?=
 SFNT2WOFFFLAGS ?=
 UFONORMALIZERFLAGS ?= -v
@@ -87,6 +88,7 @@ FONTVFLAGS ?=
 GFTOOLSFLAGS ?=
 PSAUTOHINTFLAGS ?= -vv
 PYTHONFLAGS ?= -v
+SFD2UFOFLAGS ?=
 SFDNORMALIZEFLAGS ?=
 SFNT2WOFFFLAGS ?=
 TTFAUTOHINTFLAGS ?= -v
@@ -100,6 +102,7 @@ FONTVFLAGS ?= 2> /dev/null
 GFTOOLSFLAGS ?= > /dev/null
 PSAUTOHINTFLAGS ?= 2> /dev/null
 PYTHONFLAGS ?= 2> /dev/null
+SFD2UFOFLAGS ?= 2> /dev/null
 SFDNORMALIZEFLAGS ?= 2> /dev/null
 SFNT2WOFFFLAGS ?= 2> /dev/null
 TTFAUTOHINTFLAGS ?= 2> /dev/null
@@ -112,6 +115,7 @@ FONTVFLAGS ?=
 GFTOOLSFLAGS ?=
 PSAUTOHINTFLAGS ?= -v
 PYTHONFLAGS ?=
+SFD2UFOFLAGS ?=
 SFDNORMALIZEFLAGS ?=
 SFNT2WOFFFLAGS ?=
 TTFAUTOHINTFLAGS ?=