[prefix] Add .text16.early section

[prefix] Add .text16.early section

Add a section .text16.early which is always kept inline with the
prefix.  This will allow for some code sharing between the .prefix and
.text16 sections.

Note that the simple solution of just prepending the .prefix section
to the .text16 section will not work, because a bug in Wyse Streaming
Manager server (WLDRM13.BIN) requires us to place a dummy PXENV+ entry
point at the start of .text16.

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