Changes for supporting aarch64

- Amends to the x86_64 docker file to accommodate
 changed done for adding aarch64

- Correction to install-libfdt.sh to cater for
 the case of aarch64 host platform

Signed-off-by: Raghuram Thammiraju <raghuram.thammiraju@arm.com>
Change-Id: I945cf609c45a230940bc27aa68ee0d2902444eed