release: arm: Add a GENERIC sdcard image

This produce a generic sdcard image using armv7 GENERIC kernel that
just need some u-boot (or none if the board have u-boot or a SPI flash
for example).

Reviewed by:	imp, gjb
Differential Revision:	https://reviews.freebsd.org/D16410

release/arm/GENERICSD.conf

@@ -0,0 +1,16 @@
+#!/bin/sh
+#
+# $FreeBSD$
+#
+
+EMBEDDED_TARGET_ARCH="armv7"
+EMBEDDED_TARGET="arm"
+EMBEDDEDBUILD=1
+FAT_SIZE="50m -b 1m"
+FAT_TYPE="16"
+IMAGE_SIZE="3072M"
+KERNEL="GENERIC"
+MD_ARGS="-x 63 -y 255"
+NODOC=1
+PART_SCHEME="MBR"
+export BOARDNAME="GENERICSD"