When invoking the bootstrap build in the L4RE build-system to create
a single elf-image containing all needed files to boot a scenario, don't
use the 'ENTRY' variable, but 'E' variable instead. Otherwise 'ENTRY'
might get overridden (dependent on the make-version). Moreover, using
'E' seems to be the way L4Re is expecting it has to be invoked.
Fixes#226