initramfs: setup keymapping and video for prompts

From Steve Langasek <steve.langasek@canonical.com>:
> The poorly-named 'FRAMEBUFFER' option in initramfs-tools controls
> whether the console_setup and plymouth scripts are included and used
> in the initramfs. These are required for any initramfs which will be
> prompting for user input: console_setup because without it the user's
> configured keymap will not be set up, and plymouth because you are
> not guaranteed to have working video output in the initramfs without
> it (e.g. some nvidia+UEFI configurations with the default GRUB
> behavior).

> The zfs initramfs script may need to prompt the user for passphrases
> for encrypted zfs datasets, and we don't know definitively whether
> this is the case or not at the time the initramfs is constructed (and
> it's difficult to dynamically populate initramfs config variables
> anyway), therefore the zfs-initramfs package should just set
> FRAMEBUFFER=yes in a conf snippet the same way that the
> cryptsetup-initramfs package does
> (/usr/share/initramfs-tools/conf-hooks.d/cryptsetup).

https://bugs.launchpad.net/ubuntu/+source/zfs-linux/+bug/1856408

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Kjeld Schouten <kjeld@schouten-lebbing.nl>
Signed-off-by: Steve Langasek <steve.langasek@canonical.com>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #9723

contrib/initramfs/conf-hooks.d/zfs

@@ -1,2 +1,9 @@
 # Force the inclusion of Busybox in the initramfs.
 BUSYBOX=y
+
+# Setup the keyboard mapping so passphrases can be entered correctly.
+KEYMAP=y
+
+# Require the plymouth script to guarantee working video for the passphrase
+# prompting.
+FRAMEBUFFER=y