tools/kwbimage.c: fix build on darwin

tools/kwbimage.c: fix build on darwin

kwbimage uses get_current_dir_name(3) which is a gnu extension and not
available on darwin host. Fix this by converting to portable getcwd(3)
function.

This patch fixes the following error:
---8<---
  HOSTCC  tools/kwbimage.o
tools/kwbimage.c:399:16: warning: implicit declaration of function 'get_current_dir_name' is invalid in C99 [-Wimplicit-function-declaration]
                        char *cwd = get_current_dir_name();
                                    ^
tools/kwbimage.c:399:10: warning: incompatible integer to pointer conversion initializing 'char *' with an expression of type 'int' [-Wint-conversion]
                        char *cwd = get_current_dir_name();
                              ^     ~~~~~~~~~~~~~~~~~~~~~~
2 warnings generated.
...
Undefined symbols for architecture x86_64:
  "_get_current_dir_name", referenced from:
      _image_headersz_v1 in kwbimage.o
ld: symbol(s) not found for architecture x86_64
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: Stefan Roese <sr@denx.de>
Acked-by: Stefan Roese <sr@denx.de>
[agust: fixed getcwd() return warning]
Signed-off-by: Anatolij Gustschin <agust@denx.de>