elflink: Fix boot sector booting

elflink: Fix boot sector booting

This adds missing support for booting from a boot sector file such as
.bs, .bss or .0, by re-implementing the old asm bootsec code from
core/bootsect.inc in C.

This has resulted in some external changes. We've had to make StackBuf
a global symbol because we access it directly from execute.c. Also, we
need to move dsinfo.c into MINLIBOBJS because ldlinux now needs to
reference __syslinux_derivative_info.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>