[LIBS] Make libfdt mandatory for all architectures.

Almost all architectures now and in-future will use DTB to populate
device tree in Xvisor.

Even x86 would be using it have guest templates and skeletal
device tree linked with Xvisor x86 binary. Actually, Xvisor x86
would be populating device tree from built-in DTB, ACPI tables
and Multiboot information.

This patch removes the configuration options that enables/disables
libfdt and DTC compiler.

Signed-off-by: Anup Patel <anup@brainfault.org>