Kconfig.zephyr: help text fixes

The text "need by some platforms" in various CONFIG_BUILD_OUTPUT_FOO
help is grammatically incorrect. Fix this by rewording.

Move 'config BUILD_NO_GAP_FILL' to before the list of 'config
BUILD_OUTPUT_FOO' definitions instead of sandwiching it inside.

Fix the extension of the stripped binary.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>