We currently carry a few patches to enable `search` by partition label
and uuid, but don't include the `search` module.  This commit adds the
module to enable this functionality in our grub config.

This commit also adds an additional command to our grub configs.  The
command uses the `search` module to set the `private` variable to the
location of the `BOTTLEROCKET-PRIVATE` partition.  The `private`
variable will be used to populate the grub menuentry that points to the
bootconfig file.

Adds `CONFIG_BOOT_CONFIG` to the kernel 5.10 config which enables the
use of the "Boot config" mechanism.  This allows a user to pass a
configuration file attached to the end of the initrd which is used to
extend the kernel command line at runtime.

This change adds a `grub-features` property to the supported configuration
that can be set in a variant's `Cargo.toml`.  This property is meant to gate
certain features of grub and it's config to variants that support it.  Since
grub isn't updated during OS update, it's important to ensure that variants
which are known to support a certain grub feature receive the proper grub
config.

Currently, the only supported parameter is `set-private-var`.  This
parameter means the variant has a grub config that understands how to search
for and find the BOTTLEROCKET_PRIVATE partition, and set the corresponding
`$private` variable.

This change conditionally adds the `bootconfig` parameter to the kernel
command line if `set-private-var` is added to a variants
`grub-features`.  The `bootconfig` parameter signals the kernel to look
for the Boot Configuration at the end of the specified initrd.  This
change also adds the corresponding initrd entry to the grub menuentry
which points at the bootconfig file on the `BOTTLEROCKET-PRIVATE`
partition.  Bottlerocket technically doesn't use an initrd, so this
initrd will only contain Boot Configuration data.  If the file doesn't
contain Boot Config data, the kernel logs a message but does not fail to
boot.

An empty `bootconfig.data` file is created and added to the
`BOTTLEROCKET_PRIVATE` partition to keep grub from pausing and printing
an error that the file doesn't exist.

This change organizes the kernel and init command line parameters and
moves the init parameters to the section after the "--" where they
belong.

This change adds the `set-private-var` parameter to new `grub-features`
section of the variant config for all metal variants.