kernel.bbclass: Allow initramfs to be built from a separate multiconfig

There may be a case where we want to build an initramfs image
that doesnt inherit the same DISTRO_FEATURES (or others) from
the main image being built.

For example we may want our initramfs not to inherit a certain
conf or feature, say we want to use musl for a smaller footprint,
but if we are using TCLIBC=glibc for our DISTRO (and inherently
our main image), the initramfs image would inherit that conf and
be forced to use glibc, growing in size as a side effect, currently
avoiding this is not supported.

Allow the kernel class to create a multiconfig dependency
(mcdepends) vs depends for do_bundle_initramfs and define
our INITRAMFS_IMAGE from a separate multiconfig via two
new variables:
INITRAMFS_MULTICONFIG and INITRAMFS_DEPLOY_DIR_IMAGE which
define the multiconfig where the initramfs image should be
coming from and its deploy directory respectively, these two
keep a default definition which preserves current behavior
(do_bundle_initramfs uses depends).

Example usage:
- Create and use multiconfig initramfscfg.conf and set
  TCLIBC=musl there, along with its TMPDIR.
- Add the following to our DISTRO.conf:
  INITRAMFS_MULTICONFIG = "initramfscfg"

  and set
  INITRAMFS_DEPLOY_DIR_IMAGE to the DEPLOY_DIR_IMAGE of the
  initramfscfg multiconfig (hence our main kernel will be
  able to grab it from there and bundle it).

This will result in our musl based initramfs bundled in our
main kernel and our main image to be glibc based.

(From OE-Core rev: 2d317b2685211f1b0d102705a63c0000df96f45f)

Signed-off-by: Alejandro Enedino Hernandez Samaniego <alhe@linux.microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

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