[cmdline] Add trivial logical operators to iPXE command lines

[cmdline] Add trivial logical operators to iPXE command lines

Make the "||" and "&&" operators available within iPXE commands.  For
example:

   dhcp net0 || set net0/ip 192.168.0.2

would attempt to acquire an IP address via DHCP, falling back to a
static address if DHCP fails.

As a side-effect, comments may now be appended to any line.  For
example:

  dhcp net0 || set net0/ip 192.168.0.2   # Try DHCP first, then static

Signed-off-by: Michael Brown <mcb30@ipxe.org>