bootflow: Fix build error when BOOTMETH_CROS is disabled

The bootflow testing assumes that BOOTMETH_CROS is enabled but it
might not be which leads to a build error.

aarch64-linux-gnu-ld.bfd: test/boot/bootflow.o: in function `prep_mmc_bootdev':
test/boot/bootflow.c:549:(.text.prep_mmc_bootdev+0x1c8):
    undefined reference to `_u_boot_list_2_driver_2_bootmeth_cros'

Fixes: d08db02d ("bootstd: Add a test for bootmeth_cros")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>