This patch adds support for enrolling secure boot boot keys from sd-boot.

 This patch adds support for enrolling secure boot boot keys from sd-boot.

***DANGER*** NOTE ***DANGER***

This feature might result in your device becoming soft-brick as outlined
below, please use this feature carefully.

***DANGER*** NOTE ***DANGER***

If secure-boot-enrollment is set to no, then no action whatsoever is performed,
no matter the files on the ESP.

If secure boot keys are found under $ESP/loader/keys and secure-boot-enrollment
is set to either manual or force then sd-boot will generate enrollment entries
named after the directories they are in. The entries are shown at the very bottom
of the list and can be selected by the user from the menu. If the user selects it,
the user is shown a screen allowing for cancellation before a timeout. The enrollment
proceeds if the action is not cancelled after the timeout.

Additionally, if the secure-boot-enroll option is set to 'force' then the keys
located in the directory named 'auto' are going to be enrolled automatically. The user
is still going to be shown a screen allowing them to cancel the action if they want to,
however the enrollment will proceed automatically after a timeout without
user cancellation.

After keys are enrolled, the system reboots with secure boot enabled therefore, it is
***critical*** to ensure that everything needed for the system to boot is signed
properly (sd-boot itself, kernel, initramfs, PCI option ROMs).

This feature currently only allows loading the most simple set of variables: PK, KEK
and db.

The files need to be prepared with cert-to-efi-sig-list and then signed with
sign-efi-sig-list.

Here is a short example to generate your own keys and the right files for
auto-enrollement.

`
keys="PK KEK DB"
uuid="{$(systemd-id128 new -u)}"
for key in ${keys}; do
	openssl req -new -x509 -subj "/CN=${key}/ -keyout "${key}.key" -out "${key}.crt"
	openssl x509 -outform DER -in "${key}.crt" -out "${key}.cer"
	cert-to-efi-sig-list -g "${uuid}" "${key}.crt" "${key}.esl.nosign"
done

sign-efi-sig-list -c PK.crt -k PK.key PK PK.esl.nosign PK.esl
sign-efi-sig-list -c PK.crt -k PK.key KEK KEK.esl.nosign KEK.esl
sign-efi-sig-list -c KEK.crt -k KEK.key db db.esl.nosign db.esl
`

Once these keys are enrolled, all the files needed for boot ***NEED*** to be signed in
order to run. You can sign the binaries with the sbsign tool, for example:

`
sbsign --key db.key --cert db.crt bzImage --output $ESP/bzImage
`

Example:

Assuming the system has been put in Setup Mode:

`
$ESP/loader/keys/auto/db.esl
$ESP/loader/keys/auto/KEK.esl
$ESP/loader/keys/auto/PK.esl
$ESP/loader/keys/Linux Only/db.esl
$ESP/loader/keys/Linux Only/KEK.esl
$ESP/loader/keys/Linux Only/PK.esl
$ESP/loader/keys/Linux and Windows/db.esl
$ESP/loader/keys/Linux and Windows/KEK.esl
$ESP/loader/keys/Linux and Windows/PK.esl
`

If auto-enroll is set, then the db, KEK and then PK are enrolled from the 'auto'
directory.

If not, three new boot entries are available to the user in order to enroll either the
'Linux Only', 'Linux And Windows' or 'auto' set of keys.