tools: mxsboot: Use more portable cpu_to_le32()

tools: mxsboot: Use more portable cpu_to_le32()

Currently when building mxsboot on certain machines it reports:

  HOSTCC  tools/mxsboot
  tools/mxsboot.c: In function 'mx28_create_sd_image':
  tools/mxsboot.c:560: warning: implicit declaration of function 'htole32'
  /tmp/cchLIV6q.o: In function 'main':
  mxsboot.c:(.text+0x6d8): undefined reference to 'htole32'
  mxsboot.c:(.text+0x6e7): undefined reference to 'htole32'
  mxsboot.c:(.text+0x6f6): undefined reference to 'htole32'
  mxsboot.c:(.text+0x705): undefined reference to 'htole32'
  mxsboot.c:(.text+0x711): undefined reference to 'htole32'
  /tmp/cchLIV6q.o:mxsboot.c:(.text+0x71d): more undefined references to
  'htole32' follow
  collect2: ld returned 1 exit status
  make[1]: *** [tools/mxsboot] Error 1
  make: *** [tools] Error 2

The solution is to use cpu_to_le32() instead which is more portable,
just like other U-Boot tools [1] do.

[1] http://lists.denx.de/pipermail/u-boot/2014-October/192919.html

Suggested-by: Marek Vasut <marex@denx.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Marek Vasut <marex@denx.de>