[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/prefix/dskprefix.S

 	.text
 	.section ".prefix", "ax", @progbits
 	.code16
+	.globl	start
+start:
 
 	jmp	$BOOTSEG, $go		/* reload cs:ip to match relocation addr */
 go: 

src/arch/i386/prefix/hdprefix.S

 	.section ".prefix", "awx", @progbits
 	.code16
 	.org 0
+	.globl	start
+start:
 
 	movw	$load_image, %bp
 	jmp	find_active_partition

src/arch/i386/prefix/lkrnprefix.S

 	.arch i386
 	.org	0
 	.section ".prefix", "ax", @progbits
+	.globl	start
+start:
 /* 
 	This is a minimal boot sector.	If anyone tries to execute it (e.g., if
 	a .lilo file is dd'ed to a floppy), print an error message. 

src/arch/i386/prefix/nbiprefix.S

 	.byte	0
 	.byte	0		/* No flags */
 	.word	0x0000, 0x07c0	/* Load header to 0x07c0:0x0000 */
-	.word	entry, 0x07c0	/* Start execution at 0x07c0:entry */
+	.word	start, 0x07c0	/* Start execution at 0x07c0:entry */
 	.size	file_header, . - file_header
 
 /*****************************************************************************
  * NBI entry point
  *****************************************************************************
  */
-entry:
+	.globl	start
+start:
 	/* Install iPXE */
 	call	install
 
 	int $0x19
 
 	.previous
-	.size	entry, . - entry
+	.size	start, . - start
 
 nbi_header_end:
 	.org 512

src/arch/i386/prefix/pxeprefix.S

  *****************************************************************************
  */
 	.section ".prefix", "ax", @progbits
+	.globl	start
+start:
 	jmp	$0x7c0, $1f
 1:
 	/* Preserve registers for possible return to PXE */

src/arch/i386/prefix/romprefix.S

 	.code16
 	.arch i386
 	.section ".prefix", "ax", @progbits
+	.globl	start
+start:
 	
 	.org	0x00
 romheader:

src/arch/i386/scripts/i386.lds

  *
  */
 
+ENTRY ( start )
+
 SECTIONS {
 
     /* Each section starts at a virtual address of zero.