kexec-tools: fix arm kexec failure for __NR_kexec_file_load

kexec-tools: fix arm kexec failure for __NR_kexec_file_load

Once use kexec in arm arch, it will throw out the error info as below:

Could not find a free area of memory of 0xc78e744 bytes...

This is because that kexec use kexec_file_load to load image by default
but arm doesn't support it. So add this patch to avoid use kexec_file_load
in arm platforms.

Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>