Add support for Odroid XU3

Define new flag, PLAT_EXYNOS54XX, as well as ...5422 and ...5410
Merge pull request #6 from pchubb/master

Add support for Odroid XU3
Add code for optionally stripping CPIO archives.

This is work towards repeatable builds of seL4 projects.

JIRA: CAMKES-280
cpio-strip: Enable full build warnings.
cpio-strip: Use consistent method of bailing out.
files_to_obj: Disable exit-on-error for as brief as possible.

In particular, prior to this commit the failure of cpio-strip would not be
noticed.
Add licence header.

Addressing Adrian's comments from GINCA pull request #5.
Remove reference to internal issue tracker.

As per comments from Adrian on GINCA pull request #5.
Test result of 'which cpio-strip' instead of 'set'
cpio-strip: Remove libcpio workaround.

fb50b5f4e0eedf1da877989a7a22b38ee3e77f4d of libcpio resolves the issue we were
working around.
Switch to gnu11 as the default C standard.