test: build test/boot for CONFIG_UT_BOOTSTD=n

Building sandbox_defconfig with

    CONFIG_UT_BOOTSTD=n
    CONFIG_MEASURMENT=y

results in an error:

    /usr/bin/ld: test/cmd_ut.o:(.data.rel.cmd_ut_sub+0x408):
    undefined reference to `do_ut_measurement'

Fixes: 5999ea20 ("test: Add sandbox TPM boot measurement")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>