reorganizing at91 subtargets based on sama5 soc features and this fix
below problems.
1. able to set neon flags to sama5d2 & sama5d4 subtargets.
2. fix the make clean which removes all the subtargets in bin folder.
3. able to configure kernel specific to subtarget.
4. able to set vfpu4 flags to samad3 subtargets.
Signed-off-by: Sandeep Sheriker Mallikarjun <sandeepsheriker.mallikarjun@microchip.com>
