build-linux: Allow building debian packages for Linux

To update the infra reference Linux kernel in an Ubuntu distro build,
debian packages can be built as part of the Linux build by adding a new
config flag, LINUX_BUILD_DEB_PKG. This flag enables the building of
debian packages.

To update the refinfra kernel in an Ubuntu distro, follow these steps:

1. Build a stack to get .deb packages for the Linux image and headers.
2. Boot the Ubuntu distro on a RD FVP.
3. Copy the Linux image and headers .deb packages to the Ubuntu distro
   filesystem on the FVP.
4. Run 'sudo dpkg -i <path-to-.deb-packages>'
5. Reboot the FVP to boot the Ubuntu distro with the refinfra Linux
   kernel.

Signed-off-by: Vivek Gautam <vivek.gautam@arm.com>
Change-Id: Ib4cc0713f6f0897123e4f929bb09ed85f787bfbe