If we don't have writable grubenv, recordfail doesn't work, which means our
quickboot behavior - with a timeout of 0 - leaves the user without a
reliable way to access the boot menu if they're on UEFI, because unlike
BIOS, UEFI does not support checking the state of modifier keys (i.e.
holding down shift at boot is not detectable).
Handle this corner case by always using a non-zero timeout on EFI when
save_env doesn't work.
Reuse GRUB_RECORDFAIL_TIMEOUT to avoid introducing another variable.
QUIET_BOOT is always defined: it is 1 if --enable-quiet-boot is passed
to configure, and 0 otherwise. But every CPP conditional was based on
whether it is defined or not; so the --enable-quiet-boot code paths were
in fact always enabled.
