autotools: Make petitboot autoreconfable

autotools: Make petitboot autoreconfable

The standard way to regenerate the autotools bits is with 'autoreconf',
but that doesn't work with our custom bootstrap script. We only need the
bootstrap script to generate the version in configure.ac, which we can
easily do with m4_esyscmd_s instead.

This change allows autoreconfiguration with `autoreconf -f -i`.

Includes changes proposed by Yann E. Morin <yann.morin.1998@free.fr>,
and modifications from the original patch from
Jeremy Kerr <jk@ozlabs.org>

Signed-off-by: Jeff Bailey <jeffbailey@google.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>