[build] Work around stray sections introduced by some binutils versions

Some versions of GNU ld (observed with binutils 2.36 on Arch Linux)
introduce a .note.gnu.property section marked as loadable at a high
address and with non-empty contents.  This adds approximately 128MB of
garbage to the BIOS .usb disk images.

Fix by using a custom linker script for the prefix-only binaries such
as the USB disk partition table and MBR, in order to allow unwanted
sections to be explicitly discarded.

Reported-by: Christian Hesse <mail@eworm.de>
Tested-by: Christian Hesse <mail@eworm.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/scripts/prefixonly.lds

@@ -0,0 +1,29 @@
+/* -*- ld-script -*- */
+
+/*
+ * Linker script for prefix-only binaries (e.g. USB disk MBR)
+ *
+ */
+
+SECTIONS {
+
+    .prefix 0x0 : AT ( 0x0 ) {
+	*(.prefix)
+    }
+
+    /DISCARD/ : {
+	*(.comment)
+	*(.comment.*)
+	*(.note)
+	*(.note.*)
+	*(.eh_frame)
+	*(.eh_frame.*)
+	*(.rel)
+	*(.rel.*)
+	*(.einfo)
+	*(.einfo.*)
+	*(.discard)
+	*(.discard.*)
+    }
+
+}