cmd: bootefi: localize global device paths for efi_selftest

Device paths allocated in bootefi_test_prepare() will be immediately
consumed by do_efi_selftest() and there is no need to keep them for later
use. Introduce test-specific varialbles to make it easier to move other
bootmgr functions into library directory in the next commit.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>