sandbox: Update linker scripts to ensure appended device tree is aligned

With commit 0535e46d55 ("scripts/dtc: Update to upstream version
v1.7.2-35-g52f07dcca47c") it is now a fatal error to U-Boot if our
device tree is not 8-byte aligned. In commit 85f586035d ("ARM: OMAP2+:
Pad SPL binary to 8-byte alignment before DTB") Beleswar Padhi explains
that we must have ALIGN(x) statements inside of a section to ensure that
padding is included and not simply that the linker address counter is
incremented. To that end, this patch:
- Aligns the final section before _image_binary_end (for xPL phases) by
  8-bytes by adding '. = ALIGN(8);' to the final section before the
  symbol.
- Ensure that we do have alignment by adding an ASSERT so that when not
  aligned we fail to link (and explain why).

Signed-off-by: Tom Rini <trini@konsulko.com>

arch/sandbox/cpu/u-boot-spl.lds

@@ -25,9 +25,13 @@ SECTIONS
 		*(_u_boot_sandbox_getopt_start)
 		KEEP(*(_u_boot_sandbox_getopt))
 		*(_u_boot_sandbox_getopt_end)
+		. = ALIGN(8);
 	}
 
 	_image_binary_end = .;
 }
 
 INSERT AFTER .data;
+
+ASSERT(_image_binary_end % 8 == 0, \
+       "_image_binary_end must be 8-byte aligned for device tree");