loader: Add support for grub-emu to kexec Linux menu entries

The GRUB emulator is used as a debugging utility but it could also be
used as a user-space bootloader if there is support to boot an operating
system.

The Linux kernel is already able to (re)boot another kernel via the
kexec boot mechanism. So the grub-emu tool could rely on this feature
and have linux and initrd commands that are used to pass a kernel,
initramfs image and command line parameters to kexec for booting
a selected menu entry.

By default the systemctl kexec option is used so systemd can shutdown
all of the running services before doing a reboot using kexec. But if
this is not present, it can fall back to executing the kexec user-space
tool directly. The ability to force a kexec-reboot when systemctl kexec
fails must only be used in controlled environments to avoid possible
filesystem corruption and data loss.

Signed-off-by: Raymund Will <rw@suse.com>
Signed-off-by: John Jolly <jjolly@suse.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -923,17 +923,17 @@ magic.
 @node General boot methods
 @section How to boot operating systems
 
-GRUB has two distinct boot methods. One of the two is to load an
-operating system directly, and the other is to chain-load another boot
-loader which then will load an operating system actually. Generally
-speaking, the former is more desirable, because you don't need to
-install or maintain other boot loaders and GRUB is flexible enough to
-load an operating system from an arbitrary disk/partition. However,
-the latter is sometimes required, since GRUB doesn't support all the
-existing operating systems natively.
+GRUB has three distinct boot methods: loading an operating system
+directly, using kexec from userspace, and chainloading another
+bootloader. Generally speaking, the first two are more desirable
+because you don't need to install or maintain other boot loaders and
+GRUB is flexible enough to load an operating system from an arbitrary
+disk/partition. However, chainloading is sometimes required, as GRUB
+doesn't support all existing operating systems natively.
 
 @menu
 * Loading an operating system directly::
+* Kexec::
 * Chain-loading::
 @end menu
 
@@ -959,6 +959,20 @@ use more complicated instructions. @xref{DOS/Windows}, for more
 information.
 
 
+@node Kexec
+@subsection Kexec with grub2-emu
+
+GRUB can be run in userspace by invoking the grub2-emu tool. It will
+read all configuration scripts as if booting directly (see @xref{Loading
+an operating system directly}). With the @code{--kexec} flag, and
+kexec(8) support from the operating system, the @command{linux} command
+will directly boot the target image. For systems that lack working
+systemctl(1) support for kexec, passing the @code{--kexec} flag twice
+will fallback to invoking kexec(8) directly; note however that this
+fallback may be unsafe outside read-only environments, as it does not
+invoke shutdown machinery.
+
+
 @node Chain-loading
 @subsection Chain-loading an OS