[romprefix] Provide indication of successful call to install_prealloc

The existing "iPXE starting execution" message indicates that the BEV
(or INT19) was invoked, but gives no indication on whether or not the
iPXE source was successfully retrieved (e.g. from PMM).  Split the
"starting execution message" into "starting execution...ok"; the "ok"
indicates that the main iPXE body was successfully decompressed and
relocated.

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

src/arch/i386/prefix/romprefix.S

 	movw	$prodstr, %si
 	xorw	%di, %di
 	call	print_message
-	movw	$exec_message, %si
+	movw	$exec_message_pre_install, %si
 	call	print_message
 
 	/* Store magic word on BIOS stack and remember BIOS %ss:sp */
 	call	alloc_basemem
 	call	install_prealloc
 
+	/* Print message indicating successful installation */
+	movw	$exec_message_post_install, %si
+	xorw	%di, %di
+	call	print_message
+
 	/* Set up real-mode stack */
 	movw	%bx, %ss
 	movw	$_estack16, %sp
 	int	$0x18
 	.previous
 
-exec_message:
-	.asciz	" starting execution\n"
-	.size exec_message, . - exec_message
+exec_message_pre_install:
+	.asciz	" starting execution..."
+	.size exec_message_pre_install, . - exec_message_pre_install
+exec_message_post_install:
+	.asciz	"ok\n"
+	.size exec_message_post_install, . - exec_message_post_install
 
 /* Wait for key press specified by %bl (masked by %bh)
  *