ARM: mxs: tools: Fix errno handling in strtoul() invocation

According to NOTE in strtoul(3), the errno must be zeroed before strtoul()
is called. Zero the errno. The NOTE reads as such:

  Since strtoul() can legitimately return 0 or ULONG_MAX (ULLONG_MAX for
  strtoull()) on both success and failure, the calling program should set
  errno  to  0  before the call, and then determine if an error occurred
  by checking whether errno has a nonzero value after the call.

This issue was detected on Fedora 19 with glibc 2.17 .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tom Rini <trini@ti.com>