[build] Work around stray sections introduced by some binutils versions

[build] Work around stray sections introduced by some binutils versions

Some versions of GNU ld (observed with binutils 2.36 on Arch Linux)
introduce a .note.gnu.property section marked as loadable at a high
address and with non-empty contents.  This adds approximately 128MB of
garbage to the BIOS .usb disk images.

Fix by using a custom linker script for the prefix-only binaries such
as the USB disk partition table and MBR, in order to allow unwanted
sections to be explicitly discarded.

Reported-by: Christian Hesse <mail@eworm.de>
Tested-by: Christian Hesse <mail@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>