cbfstool: Fix removing and adding file with same name

Currently, cbfstool regressed that removing a file from CBFS the space
is marked as empty but the filename is still shown, preventing adding a
file with the same name again. [1]

```
$ echo a > a
$ echo b > b
$ ./util/cbfstool/cbfstool  test.rom create -m x86 -s 1024
Created CBFS (capacity = 920 bytes)
$ ./util/cbfstool/cbfstool test.rom add -f a -n a -t raw
$ ./util/cbfstool/cbfstool test.rom add -f b -n b -t raw
$ cp test.rom test.rom.original
$ ./util/cbfstool/cbfstool test.rom remove  -n
$ diff -up <(hexdump -C test.rom.original) <(hexdump -C test.rom)
--- /dev/fd/63  2015-08-07 08:43:42.118430961 -0500
+++ /dev/fd/62  2015-08-07 08:43:42.114430961 -0500
@@ -1,4 +1,4 @@
-00000000  4c 41 52 43 48 49 56 45  00 00 00 02 00 00 00 50  |LARCHIVE.......P|
+00000000  4c 41 52 43 48 49 56 45  00 00 00 02 ff ff ff ff  |LARCHIVE........|
 00000010  00 00 00 00 00 00 00 28  61 00 00 00 00 00 00 00  |.......(a.......|
 00000020  00 00 00 00 00 00 00 00  61 0a ff ff ff ff ff ff  |........a.......|
 00000030  ff ff ff ff ff ff ff ff  ff ff ff ff ff ff ff ff  |................|
$ ./util/cbfstool/cbfstool test.rom add -f c -n c -t raw

$ ./util/cbfstool/cbfstool test.rom print
test.rom: 1 kB, bootblocksize 0, romsize 1024, offset 0x0
alignment: 64 bytes, architecture: x86

Name                           Offset     Type         Size
c                              0x0        raw          2
b                              0x40       raw          2
(empty)                        0x80       null         792
```

So it is “deteled” as the type changed. But the name was not changed to
match the *(empty)* heuristic.

So also adapt the name when removing a file by writing a null byte to
the beginning of the name, so that the heuristic works. (Though remove
doesn't really clear contents.)

```
$ ./util/cbfstool/cbfstool test.rom remove  -n c
$ ./util/cbfstool/cbfstool test.rom print
test.rom: 1 kB, bootblocksize 0, romsize 1024, offset 0x0
alignment: 64 bytes, architecture: x86

Name                           Offset     Type         Size
(empty)                        0x0        null         2
b                              0x40       raw          2
(empty)                        0x80       null         792
```

[1] http://www.coreboot.org/pipermail/coreboot/2015-August/080201.html

Change-Id: I033456ab10e3e1b402ac2374f3a887cefd3e5abf
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/11632
Tested-by: build bot (Jenkins)
Tested-by: Raptor Engineering Automated Test Stand <noreply@raptorengineeringinc.com>

util/cbfstool/cbfs_image.c

@@ -914,6 +914,7 @@ int cbfs_merge_empty_entry(struct cbfs_image *image, struct cbfs_file *entry,
 			   unused void *arg)
 {
 	struct cbfs_file *next;
+	uint8_t *name;
 	uint32_t type, addr, last_addr;
 
 	type = ntohl(entry->type);
@@ -921,6 +922,9 @@ int cbfs_merge_empty_entry(struct cbfs_image *image, struct cbfs_file *entry,
 		// Ready to be recycled.
 		type = CBFS_COMPONENT_NULL;
 		entry->type = htonl(type);
+		// Place NUL byte as first byte of name to be viewed as "empty".
+		name = (void *)&entry[1];
+		*name = '\0';
 	}
 	if (type != CBFS_COMPONENT_NULL)
 		return 0;