build/install: Add USB boot option

build/install: Add USB boot option

Use isohybrid to add MBR for booting via BIOS
from USB flash drives

Also, Makefile "$(wildcard ubuntu-builder/*)" does not expand
to what is expected (does not include ubuntu-builder/Dockerfile),
as it uses a previous cached value.
Check this link for an explanation:
http://stackoverflow.com/questions/41470676/makefile-variable-expansion-with-wildcard-when-rule-creates-new-file

JIRA: FUEL-236

Change-Id: I9c6521a382917822ce930ddf76391c5f3c3a9cb7
Signed-off-by: Matei Valeanu <Matei.Valeanu@enea.com>