arm: Use image_setup_linux() instead of local code

Use the common FDT setup function that is now available in image. Move
the FDT-specific code to a new bootm-fdt.c and remove unused headers
from bootm.c.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/arm/lib/Makefile b/arch/arm/lib/Makefile
index 6ae161a..5b2cb61 100644
--- a/arch/arm/lib/Makefile
+++ b/arch/arm/lib/Makefile
@@ -45,6 +45,7 @@
 COBJS-y += bss.o
 
 COBJS-y	+= bootm.o
+COBJS-$(CONFIG_OF_LIBFDT) += bootm-fdt.o
 COBJS-$(CONFIG_SYS_L2_PL310) += cache-pl310.o
 SOBJS-$(CONFIG_USE_ARCH_MEMSET) += memset.o
 SOBJS-$(CONFIG_USE_ARCH_MEMCPY) += memcpy.o