aemfvp-a-rme: add utils for supporting distro image preparation

Changes include
- adding script distro_kernel_debs_copy for mounting the linux filesystem
  of the distro image and copying the linux debs to /boot path.
- distro_dpkg_update for running sudo dpkg -i <debs> on booted fvp distro
  image to replace the stock kernel in distro image.
- rebuilding buildroot with rebuild_buildroot to repack the updated distro
  image in host's filesystem.

Signed-off-by: nitin sharma <nitin.sharma3@arm.com>
Change-Id: Id013face7e7d0061e1aced4c35c8ab20f4c183d7