u-boot-boundary-fw-utils: Limit the target to imx machines only

It fails to build for non-arm machines anyway since it does not provide the needed env file for anything other than arm and aarch64

Let cross version build too

---
 recipes-bsp/u-boot/u-boot-boundary-fw-utils_2018.07.bb | 3 +++
 1 file changed, 3 insertions(+)

index 56ba151..0a7697a 100644
@@ -42,3 +42,6 @@ uboot_fw_utils_cross() {
 RPROVIDES_${PN} += "u-boot-fw-utils"
 
 BBCLASSEXTEND = "cross"
+
+COMPATIBLE_MACHINE = "(imx)"
+COMPATIBLE_MACHINE_class-cross = "(.*)"

2.19.1