Add support for loading modules in tiny-initramfs. The support is
extremely simple, it just looks for a file /modules in the initramfs
image, where each line should be the absolute path (within the
initramfs image) to the module file name, followed optionally by a
space and the options.

No dependency resolution is performed, the modules are loaded in the
order they are specified in the /modules file. The construction of the
initramfs image should take care of the proper order of the modules and
adding all required dependencies.

If a module cannot be loaded, an error message will be shown, but
tiny-initramfs will still try to mount the root file system.

Install the binary as init (in pkglibexecdir) instead of tiny_initramfs
and add support for variants (init-$VARIANT) that may be passed to
configure. That way, builds of tiny-initramfs with different compile
time options may be installed simultaneously on a given system.

Documentation/filesystems/ramfs-rootfs-initramfs.txt says we should do
that. Note that this was not a huge problem before, as our initramfs is
really small anyway.

Switch to an automake-based build system, and allow certain features to
be optional, so that the initrd.img size may be reduced further.

Update the documentation.

Do some changes that reduce the code size a bit.

Add basic NFSv4 mounting support. This increases code size a bit (will
thus be made optional later), but it now supports mounting NFSv4 shares
directly - as long as sec=sys is used and no idmapper is required.

The kernel must have compiled-in support for NFSv4 for this to work.

To help with debugging of the initramfs, add #ifdef'd debug statements.
When compiled with CPPFLAGS="-DDEBUG_INITRAMFS" the initramfs will now
be verbose - and sleep for 5s at the end before executing init.

Instead of simply looking for the device by name, if either UUID= or
0xMAJMIN is specified for the root (or /usr) device, loop through all
devices in /dev and try to find the matching device.

For UUID= we parse filesystem superblocks for ext[234], xfs and btrfs.
(Other filesystems can easily be added for this.)

As pointed out by Macro d'Itri, this technically is an initramfs and
not an initrd, so rename it to reflect that.