conf/machine/include: enable hardfloat by default for ARMv6 and above

conf/machine/include: enable hardfloat by default for ARMv6 and above

Defaulting to softfp probably isn't the best choice anymore,
especially as there are now ARM BSP layers which leave DEFAULTTUNE
entirely up to the distro:

  https://lists.yoctoproject.org/pipermail/yocto/2017-February/034637.html

Also add 't' to the ARMv7 default DEFAULTTUNEs, since there's no
clear reason to default to ignoring ARM_INSTRUCTION_SET for ARMv7.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>