toolchain: move elf2flt option inclusion to toolchain-buildroot/Config.in

The only remaining thing in toolchain-buildroot/Config.in.2 is the
inclusion of the elf2flt option. It doesn't really make sense to have
a separate Config.in file for that, so let's move this to
toolchain-buildroot/Config.in.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>