flashrom: fix static build with uClibc

Define HAVE_STRNLEN to avoid local strnlen() definition.

Fixes:
http://autobuild.buildroot.net/results/7dc/7dc4298e3a07c73e03f70205516d68a0f4c2d297/
http://autobuild.buildroot.net/results/e36/e362848eb45f6b8100131361e6e5faa546f0bbd8/
http://autobuild.buildroot.net/results/69e/69ef10ec710f418b4d10c1edb4f2ce2e49b522bf/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/flashrom/flashrom.mk

@@ -12,7 +12,8 @@ FLASHROM_LICENSE = GPL-2.0+
 FLASHROM_LICENSE_FILES = COPYING
 
 define FLASHROM_BUILD_CMDS
-	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) -C $(@D)
+	$(TARGET_MAKE_ENV) $(MAKE) $(TARGET_CONFIGURE_OPTS) \
+		CFLAGS="$(TARGET_CFLAGS) -DHAVE_STRNLEN" -C $(@D)
 endef
 
 define FLASHROM_INSTALL_TARGET_CMDS