drivers/mtd_flashpage: Should fail on compilation if slot_aux is used while feature is not present

[FlapKap]

Contribution description

since CONFIG_SLOT_AUX_LEN is only supported on platforms where MODULE_PERIPH_FLASHPAGE_AUX is provided, it should also only successfuly compile on those platforms. Thats not the case right now, as there only is a check if CONFIG_SLOT_AUX_LEN is defined. This PR makes sure we fail the compilation if we have CONFIG_SLOT_AUX_LEN without MODULE_PERIPH_FLASHPAGE_AUX

Testing procedure

Compile the tests/drivers/mtd_flashpage without the #ifdef MODULE_PERIPH_FLASHPAGE_AUXs and check that it fails

Issues/PRs references