tools: kwbimage: Fix compilation without CONFIG_SYS_U_BOOT_OFFS

tools: kwbimage: Fix compilation without CONFIG_SYS_U_BOOT_OFFS

The CONFIG_SYS_U_BOOT_OFFS option may be defined as empty string.
In this case it causes compilation error:

    tools/kwbimage.c: In function ‘image_headersz_v1’:
    tools/kwbimage.c:1002:39: error: expected expression before ‘)’ token
      if (headersz > CONFIG_SYS_U_BOOT_OFFS) {
                                           ^
    tools/kwbimage.c:1006:41: error: expected expression before ‘)’ token
        (int)headersz, CONFIG_SYS_U_BOOT_OFFS);
                                             ^
    tools/kwbimage.c:1011:35: error: expected expression before ‘;’ token
      headersz = CONFIG_SYS_U_BOOT_OFFS;
                                       ^
    make[1]: *** [scripts/Makefile.host:112: tools/kwbimage.o] Error 1
    make: *** [Makefile:1822: tools] Error 2

Check whether the value of CONFIG_SYS_U_BOOT_OFFS is really set.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>