ARM: mxs: tools: Use mkimage for BootStream generation

Now that mkimage can generate an BootStream for i.MX23 and i.MX28,
use the mkimage as a default tool to generate the BootStreams instead
of the elftosb tool. This cuts out another obscure dependency.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Cc: Otavio Salvador <otavio@ossystems.com.br>
Cc: Stefano Babic <sbabic@denx.de>
diff --git a/arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg b/arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg
index ea772f0..676f5c8 100644
--- a/arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg
+++ b/arch/arm/cpu/arm926ejs/mxs/mxsimage.mx28.cfg
@@ -1,8 +1,8 @@
 SECTION 0x0 BOOTABLE
  TAG LAST
- LOAD     0x0        spl/u-boot-spl.bin
+ LOAD     0x0        OBJTREE/spl/u-boot-spl.bin
  LOAD IVT 0x8000     0x14
  CALL HAB 0x8000     0x0
- LOAD     0x40000100 u-boot.bin
+ LOAD     0x40000100 OBJTREE/u-boot.bin
  LOAD IVT 0x8000     0x40000100
  CALL HAB 0x8000     0x0