doc/develop/distro.rst: Better document upstream definition of extlinux.conf

doc/develop/distro.rst: Better document upstream definition of extlinux.conf

First, the "Boot Loader Specification" link has moved to a new location,
so link to that directly. Second, that link does not document as much of
the extlinux.conf format as I recall the old version doing at least.
However, the Syslinux Project wiki is the current location of the documentation
linked to in doc/README.pxe and also has a reference for SYSLINUX. Link
to both of these.

Signed-off-by: Tom Rini <trini@konsulko.com>