net: tftp: remove explicit efi configuration dependency

Now it is clear that the feature actually depends on efi interfaces,
not "bootefi" command. efi_set_bootdev() will automatically be nullified
if necessary efi component is disabled.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>