The online documentation (e.g. http://ipxe.org/cmd/ifopen), though not
yet complete, is far more comprehensive than could be provided within
the iPXE binary.  Save around 200 bytes (compressed) by removing the
command descriptions from the interactive help, and instead referring
users directly to the web page describing the relevant command.

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

Total saving: 111 bytes.

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

There are several points in the iPXE codebase where
list_for_each_entry() is (ab)used to extract only the first entry from
a list.  Add a macro list_first_entry() to make this code easier to
read.

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

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