am33x-cm3: available only for ELF builds

am33x-cm3: available only for ELF builds

The am33x-cm3 build system does some objcopy based binary file
manipulation that assumes the compiler for the platform generates ELF
file. While it's true for most ARM platforms, and especially the AM335x
based ones, it is no longer always true since we added support for
Cortex-M. Therefore, this package needs to depend on BR2_BINFMT_ELF.

Fixes:

  http://autobuild.buildroot.net/results/05f46ab28cbfc587e2e031bf38fb975447f8bab6/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>