build: add support for x86_64 host

build: add support for x86_64 host

For x86_64 host local build, this patch adds prebuilt mkimage from
u-boot and local build script files uses the prebuilt mkimage as
host architecture. The mkimage requires dtc also, but it is already
in scripts/dtc and built as host tool during kernel build,so dtc is
not required prebuilt files.

Change-Id: Id3f734ee39cc1e639bcaff2a7e49ad6d6c66665a
Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>