introduce environment variables for all qemu-user options

(Edits by Riku Voipio to apply to current HEAD)

Rework option parsing code for linux-user in a table-driven manner to allow
environment variables for all commandline options.

Also generate usage() output from option table.

Fix complains from checkpatch.pl, also have envlist global

Signed-off-by: Johannes Schauer <j.schauer@email.de>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>
