[build] Ensure an entry point symbol exists in all builds

Commit 623469d ("[build] Eliminate unused sections at link-time")
introduced a regression in several build formats, in which the prefix
would end up being garbage-collected out of existence.  Fix by
ensuring that an entry symbol exists in each possible prefix, and is
required by the linker script.

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

src/arch/i386/scripts/i386.lds

@@ -5,6 +5,8 @@
  *
  */
 
+ENTRY ( start )
+
 SECTIONS {
 
     /* Each section starts at a virtual address of zero.