linux-*: enable NLS_ASCII for kernels that support EFI boot (MR 5136)

These kernels must have NLS_ASCII set after bb6d7a05, or else they
cannot mount the ESP as /boot and boot fails. This kconfig option will
be added to the pmb efi kconfig check so that this is caught in the
future pre-merge.

fixes 3909b1db and bb6d7a05

[ci:skip-build] already built successfully in CI