u-boot-mkimage: fix nativesdk build

If building for nativesdk the wrong rss sysroot is used leading the
following error message.

| ERROR: oe_runmake failed
| In file included from tools/imximage.c:13:0:
| include/image.h:1024:27: fatal error: openssl/evp.h: No such file or directory
|  # include  <openssl/evp.h>

Tools needed on the build host (script/basic/fixdep) and code compiled
for the SDK machine are both built with the build host's compiler,
leading to additinal errors.

Adding CROSS_COMPILE="${HOST_PREFIX}" and using the cross-compiler for
the SDK_ARCH fixes the build error.
The resulting binary in the SDK is working.

(From OE-Core rev: 5d3df78367be0afbfe001b4fa776a98a82e6ce54)

Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-bsp/u-boot/u-boot-mkimage_2017.01.bb

@@ -7,7 +7,7 @@ DEPENDS = "openssl"
 
 EXTRA_OEMAKE_class-target = 'CROSS_COMPILE="${TARGET_PREFIX}" CC="${CC} ${CFLAGS} ${LDFLAGS}" HOSTCC="${BUILD_CC} ${BUILD_CFLAGS} ${BUILD_LDFLAGS}" STRIP=true V=1'
 EXTRA_OEMAKE_class-native = 'CC="${BUILD_CC} ${BUILD_CFLAGS} ${BUILD_LDFLAGS}" HOSTCC="${BUILD_CC} ${BUILD_CFLAGS} ${BUILD_LDFLAGS}" STRIP=true V=1'
-EXTRA_OEMAKE_class-nativesdk = 'CC="${BUILD_CC} ${BUILD_CFLAGS} ${BUILD_LDFLAGS}" HOSTCC="${BUILD_CC} ${BUILD_CFLAGS} ${BUILD_LDFLAGS}" STRIP=true V=1'
+EXTRA_OEMAKE_class-nativesdk = 'CROSS_COMPILE="${HOST_PREFIX}" CC="${CC} ${CFLAGS} ${LDFLAGS}" HOSTCC="${BUILD_CC} ${BUILD_CFLAGS} ${BUILD_LDFLAGS}" STRIP=true V=1'
 
 do_compile () {
 	oe_runmake sandbox_defconfig