[script] Allow for backslash continuation of script lines

Allow long script lines to be broken up using backslash continuation.
For example:

   choose --default linux --timeout 3000 os \
   	  && goto boot_${os} || goto cancelled

Requested-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[velocity] Rewrite VIA Velocity driver

Signed-off-by: Adrian Jamróz <adrian.jamroz@gmail.com>
Modified-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Thomas Miletich <thomas.miletich@gmail.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>