Add autotools configuration.

This patch adds a basic auto-tools configuration environment for the
bootwrapper consisting of the configure.ac file and the Makefile.am
file.

The configure.ac file includes several options and checks:
  - Ensure that an AArch64 compiler is used.
  - Add the mandatory --with-kernel-dir option that sets KERN_DIR.
  - Check that the correct base dtb file exists in KERN_DIR.
  - Add an option, --with-initrd, that allows a user to specify an
    initrd file to embed in the image.
  - Check for a working dtc and set DTC to the full-path to the
    executable.
  - Create the necessary symbolic links to the relevant kernel files.

The Makefile.am file includes several changes:

- Use a prebuilt dtb rather than a dts, which is decompiled to inject
  the chosen node.
- Use the configured compiler tool names.
- Use the configured SED program rather than assuming it is in PATH.

Signed-off-by: Matthew Leach <matthew.leach@arm.com>
[Mark: use dtb, add options, fix style issues, remove src/]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>