[legal] Relicense files under GPL2_OR_LATER_OR_UBDL

Several of the assembly files in arch/i386/prefix were missed by the
automated relicensing tool due to missing licence declarations, code
dating back to the initial git revision, etc.  Manual review shows
that these files may be relicensed.

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

src/arch/i386/prefix/hdprefix.S

-FILE_LICENCE ( GPL2_OR_LATER )
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 
 	.text
 	.arch i386

src/arch/i386/prefix/lkrnprefix.S

-FILE_LICENCE ( GPL_ANY )
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 
 #define BZI_LOAD_HIGH_ADDR 0x100000
 

src/arch/i386/prefix/mbr.S

+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+
 	.text
 	.arch i386
 	.section ".prefix", "awx", @progbits

src/arch/i386/prefix/nbiprefix.S

+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+
 	.text
 	.arch i386
 	.code16

src/arch/i386/prefix/nullprefix.S

+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+
 	.org	0
 	.text
 	.arch i386

src/arch/i386/prefix/pxeprefix.S

-FILE_LICENCE ( GPL2_OR_LATER )
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
 
 #define PXENV_UNDI_SHUTDOWN		0x0005
 #define	PXENV_UNDI_GET_NIC_TYPE		0x0012

src/arch/i386/prefix/usbdisk.S

+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL )
+
 	.text
 	.arch i386
 	.section ".prefix", "awx", @progbits