stand/i386: build a specific loader for PXE boot

[khorben_defora.org]

Summary

The current PXE loader does not work since it became too big. This change creates a dedicated loader for use when booting over PXE, with support removed for the FAT and EXT2 filesystems, as well as for GZIP and BZIP2 compression. This new loader still defaults to the same interpreter as the regular loader.

It is then possible to boot the kernel over TFTP or NFS again.

This is meant as a possible alternative to D41750, and relevant to PR #257018.

Test Plan

In /usr/local/etc/dhcpd.conf: (isc-dhcp44-server package)

next-server 192.168.2.1;
filename "FreeBSD/boot/pxeboot";
option root-path "nfs://192.168.2.1/tftpboot/FreeBSD/amd64";

In /etc/inetd.conf:

tftp  dgram   udp     wait    root    /usr/libexec/tftpd      tftpd -l -s /tftpboot

In /tftpboot/FreeBSD/boot:

-rw-r--r--  1 root    wheel   415744 Sep  5 21:11 pxeboot

[imp]

Maybe D42416 makes this obsolete?