Make pxeboot use rootpath option in tftp mode

[bapt]

Summary

Instead on enforcing a hierarchy on the tftp server, use rootpath
to prefix all tftp open commands so the default /boot can be put at any level of
the hierarchy or the tftp server.

While here drop the specific pxeboot.4th default rc file requirements as the
vanilla content of /boot can now be used

[bapt]

pxe http is not for this review yet :)

[jhb]

Can you explain a bit more of the before and after effect? In particular, I'm not sure what the old pxe_default_rc() was doing. (This is probably ps@ code)

[bapt]

In D4590#96780, @jhb wrote:

Can you explain a bit more of the before and after effect? In particular, I'm not sure what the old pxe_default_rc() was doing. (This is probably ps@ code)

Before that change:
when booting pxeboot was looking for a pxeboot.4th (pxe_default_rc) file at the same location as the pxeboot itself on the tftp server (file which is not provided in our sources). if that file was containing any "include /boot/something" then it was looked up at the root of the tftpboot server, meaning we could not use the regular files we do have in /boot.

Now when pxeboot start it is looking for the regular /boot/boot.4th and /or loader.rc, it respects the root-path provided via the dhcp server, meaning that tftp loader works exactly the same as nfs loader so one can just drop the content of the /boot anywhere on the tftp server everything will just work.

if root-path is provided it is prepended (like for nfs loader) to any open fonction meaning "include /boot/something" will actually make a get /rootpath/boot/something on the tftp server.

If no root-path is provided then it will look for /boot/something at the root of the tftp server like before. The only behaviour change is instead of looking for a pxeboot.4th like it did before it will now look for the same files as the other loaders.

I hope this clarifies a bit more what this patch does

[rpokala]

lib/libstand/tftp.c

429

If rootpath ends with a '/', tftpfile->path will have a "//" in it; is that okay?

sys/boot/i386/loader/main.c

204

It would be nice if you could continue to use pxeboot.4th, for those who have it.

[imp]

lib/libstand/tftp.c

115

MAXPATHLEN is a posix concept. And it's a constant for the 'host' we're building on, not the remote TFTP server. The RFC doesn't seem to list a maximum length...

[bapt]

lib/libstand/tftp.c

429

yes it is ok

sys/boot/i386/loader/main.c

204

Well I can add a pxeboot.4th concept, which if it fails, falls back on a normal boot process, but the behaviour would be a bit different than before because pxeboot.4th would be looked up in root-path instead of "path" of the pxeboot file.

[ngie]

lib/libstand/tftp.c

48–51

Please replace sys/types.h with sys/param.h per style(9)

[bapt]

Do not use MAXPATHLEN

[bapt]

lib/libstand/tftp.c

115

Ok dropped

432

Now it is a bit cleaner as I only add a / if rootpath does not end with a / or path does not start with a /

sys/boot/i386/loader/main.c

204

adding pxeboot.4th compat is more complicated that it looks like, as for now pxeboot.4th completly bypass all other includes done in the loader. (aka prevents others to load)

[bapt]

lib/libstand/tftp.c

48–51

I have removed the need of sys/param.h

[rpokala]

Looks okay to me.