scripts/qemu-binfmt-conf.sh: re-factor and clean-up

I was looking to set-up for development but found the script made some
hard-coded assumptions. It doesn't seem the script is used by the
distros but if it had a little more love maybe it would be ;-)

* Add usage() instructions
* Move all registering to a single function
* Check for existence of executable qemu before registering
* Add -d (devel) mode for subscribing in-src tree binaries
* error handling when setting binfmt fails
* add support for aarch64
target-arm: A64: remove redundant store

There is not much point storing the same value twice in a row.

Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
.travis.yml: re-instate make check on all targets (EXP)

Peter did note that we should be running check more frequently to expose
any potential determinism problems. I'll experiment for now on my branch
and see how that works out.