u-boot-fw-utils: Skip package if UBOOT_MACHINE is unset

The UBOOT_MACHINE variable needs to be set so the firmware utils can
know about some configuration settings which are board dependent. This
patch ensures the package is skipped in case UBOOT_MACHINE is unset
thus avoid its build in 'bitbake world' builds for incompatible
machines.

Fixes [YOCTO: #5223]

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>