boot/at91bootstrap3: migrate to pkg-kconfig

In at91bootstrap3, defconfig files are spread out in various
sub-directories, so we can't easily construct the path to those
defconfig files.

As it turns out, there are no two defconfig files named the same; they
all contain at least the board name, plus some extra info. And there is
always only one directory depth below board/ where all defconfig files
are to be found.

Also, at91bootstrap3's own Makefile relies on this single-match as it
runs a 'find' command to locate the defconfig file.

So, even though we do not know the path to the defconfig file, we can
construct a shell-glob quite easily (in the hope that there will never
be more than one directory depth).

Signed-off-by: Mauro Condarelli <mc5686@mclink.it>
[yann.morin.1998@free.fr:
  - remove custom _CONFIGURE_CMDS, handled by pkg-kconfig  (Thomas)
  - remove redundant intermediate variable
  - fix using internal defconfig files
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>