Made network-bridge script work on ALL systems missing ifup/down.

Made network-bridge script work on ALL systems missing ifup/down.
This is done by defining ifup/down both to 'false' when they're
missing; then instead of attempting to parse the IP params from
the kernel command line, we get them from 'ip' instead.  So this
works even with (for example) ip=dhcp on the kernel command line.

Also made a few minor syntax changes (in particular, replaced "=="
with "=", and ">&foo" with ">foo 2>foo") so this works with more
limited shells.

Signed-off-by: Ben Thomas <bjthomas3@gmail.com>