u-boot-tools: fix compile error

u-boot-tools: fix compile error

It uses sandbox_defconfig to produce u-boot tools. But EFI is only
supported by arm and x86, then it fails to run task do_compile on other
arches:

| include/config_distro_bootcmd.h:267:3: error: #error "sandbox EFI
| support is only supported on ARM and x86"

Only enable EFI support for u-boot-tools on x86 and arm to fix the issue.

Signed-off-by: Kai Kang <kai.kang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>