security-interface-extension: add Buildroot for rootfs

This patch adds support for Buildroot for building the rootfs
and Linux kernel.

This means the following builds are removed because they are
covered by Buildroot:
   busybox
   linux
   fwts

-get_source.sh is updated to get the Buildroot source and apply
 several updates
   -add a new arm_systemready_defconfig
   -add a busybox.config to enable features needed by the ACS
   -add a Linux kernel.config to enable features needed by the ACS
   -copy a rootfs overlay with files needed in the rootfs
      -updated inittab that does not prompt for password
      -add the .ini file from the bbr-acs which has the fwts tests
       to run
      -add a init.d script to mount the securityfs and run the
       security interface extension tests

-tpm2-tools is built, and this package was updated to 5.1.1
 to pick up the event log dump tool

Signed-off-by: Stuart Yoder <stuart.yoder@arm.com>