cbfstool: Fix ability to add files at offsets near the end of empty spaces

Because cbfs_add_entry_at() previously *assumed* it would have to create a
trailing empty entry, it was impossible to add files at exact offsets close
enough to the end of an existing empty entry that they occupied the remainder
of its space. This addresses the problem by skipping the step of creating the
trailing empty entry if doing so would place it at the start offset of whatever
already followed the original empty section.

BUG=chromium:473511
TEST=Run the following commands:
$ ./cbfstool test.image create -s 0x100000 -m arm
$ dd if=/dev/zero of=twok.bin bs=1 count=2048
$ ./cbfstool test.image add -t 0x50 -f twok.bin -n at_end -b 0xff7c0
$ ./cbfstool test.image add -t 0x50 -f twok.bin -n near_end -b 0xfef80
$ ./cbfstool test.image print
There shouldn't be any assertions, and the output should be:
test.image: 1024 kB, bootblocksize 0, romsize 1048576, offset 0x40
alignment: 64 bytes, architecture: arm

Name                           Offset     Type         Size
(empty)                        0x40       null         1044184
near_end                       0xfef40    raw          2048
at_end                         0xff780    raw          2048
BRANCH=None

Change-Id: Ic8a6c3dfa4f82346a067c0804afb6c5a5e89e6c8
Signed-off-by: Sol Boucher <solb@chromium.org>
Original-Commit-Id: 1bbd353fddc818f725e488e8f2fb6e967033539d
Original-Change-Id: I15d25df80787a8e34c2237262681720203509c72
Original-Signed-off-by: Sol Boucher <solb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/263809
Original-Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Original-Reviewed-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/9938
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

util/cbfstool/cbfs_image.c

@@ -456,8 +456,10 @@ static int cbfs_add_entry_at(struct cbfs_image *image,
 	// Process buffer AFTER entry.
 	entry = cbfs_find_next_entry(image, entry);
 	addr = cbfs_get_entry_addr(image, entry);
-	assert(addr < addr_next);
+	if (addr == addr_next)
+		return 0;
 
+	assert(addr < addr_next);
 	if (addr_next - addr < min_entry_size) {
 		DEBUG("No space after content to keep CBFS structure.\n");
 		return -1;