sd-boot: add way to disable the 100ms delay when timeout=0

Currently we have a 100ms delay which allows for people to enter/show
the boot menu even when timeout is set to zero.

In a handful of cases, that may not be needed - both in terms of access
policy, as well as latency.

For example: the option to provide the boot menu may be hidden behind an
"expert only" UX in the OS, to avoid end users from accidentally
entering it.

In addition, the current 100ms input polling may cause unexpected
additional delays in the boot. Some example numbers from my SteamDeck:

 - boot counting/rename/flush doubles 300us -> 600us
 - seed/hash setup doubles 900us -> 1800us
 - kernel/image load gets ~40% slower 107ms -> 167ms

It's not entirely clear why the UEFI calls gets slower, nevertheless the
information in itself proves useful.

This commit introduces a new option "menu-disabled", which omits the
100ms delay. The option is documented throughout the manual pages as
well as the Boot Loader Specification.

v2:
 - use STR_IN_SET

v3:
 - drop erroneous whitespace

v4:
 - add a new LoaderFeature bit,
 - don't change ABI keep TIMEOUT_* tokens the same
 - move new token in the 64bit range, update API and storage for it
 - change inc/dec behaviour to TIMEOUT_MIN : TIMEOUT_MENU_FORCE
 - user cannot opt-in from sd-boot itself, add assert_not_reached()

v5:
 - s/Menu disablement control/Menu can be disabled/
 - rewrap comments to 109
 - use SYNTHETIC_ERRNO(EOPNOTSUPP)

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>

docs/BOOT_LOADER_INTERFACE.md

@@ -35,8 +35,9 @@ variables. All EFI variables use the vendor UUID
   non-numeric string values are also accepted. A value of `menu-force`
   will disable the timeout and show the menu indefinitely. If set to `0` or
   `menu-hidden` the default entry is booted immediately without showing a menu.
-  The boot loader should provide a way to interrupt this by for example
-  listening for key presses for a brief moment before booting.
+  Unless a value of `menu-disabled` is set, the boot loader should provide a
+  way to interrupt this by for example listening for key presses for a brief
+  moment before booting.
 
 * Similarly, the EFI variable `LoaderConfigTimeoutOneShot` contains a boot menu
   timeout for a single following boot. It is set by the OS in order to request
@@ -80,6 +81,7 @@ variables. All EFI variables use the vendor UUID
   * `1 << 4` → The boot loader supports boot counting as described in [Automatic Boot Assessment](AUTOMATIC_BOOT_ASSESSMENT.md).
   * `1 << 5` → The boot loader supports looking for boot menu entries in the Extended Boot Loader Partition.
   * `1 << 6` → The boot loader supports passing a random seed to the OS.
+  * `1 << 13` → The boot loader honours `menu-disabled` option when set.
 
 * The EFI variable `LoaderSystemToken` contains binary random data,
   persistently set by the OS installer. Boot loaders that support passing