arm: add _end symbol to bootblock.ld

It's helpful to view program size by inspecting the symbols.
_start and _end exist on romstage and ramstage. In order to
be consistent add _end for bootblock too.

BUG=None
BRANCH=None
TEST=Built and noted bootblock has _end symbol.

Change-Id: I7f0b4dd4078c7d23c70949563b4c3f4df9e66142
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/210832
Reviewed-by: Furquan Shaikh <furquan@chromium.org>

src/arch/arm/bootblock.ld

@@ -47,6 +47,7 @@ SECTIONS
 		*(.bss.*);
 		*(.sbss);
 		*(.sbss.*);
+		_end = .;
 	} : to_load = 0xff
 
 	preram_cbmem_console = CONFIG_CBMEM_CONSOLE_PRERAM_BASE;