[build] Use unique entry symbols for each prefix

Some binutils versions will drag in an object to satisfy the entry
symbol; some won't.  Try to cope with this exciting variety of
behaviour by ensuring that all entry symbols are unique.

Remove the explicit inclusion of the prefix object on the linker
command line, since the entry symbol now provides all the information
needed to identify the prefix.

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

src/arch/i386/prefix/mromprefix.S

@@ -30,6 +30,7 @@ FILE_LICENCE ( GPL2_OR_LATER )
 #define PCI_BAR_EXPROM			0x30
 
 #define ROMPREFIX_EXCLUDE_PAYLOAD 1
+#define _rom_start _mrom_start
 #include "romprefix.S"
 
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