boot/at91bootstrap3: add support for at91bootstrap 4.x series

boot/at91bootstrap3: add support for at91bootstrap 4.x series

The project at https://github.com/linux4sam/at91bootstrap was until
now releasing 3.x versions, which were packaged using
boot/at91bootstrap3/ in Buildroot. Microchip has now started a new
branch of at91bootstrap, called 4.x, which will only support the
following devices: sam9x60, sama5d2, sama5d3, sama5d4, sama7g5. A
number of older devices from Microchip will only be supported by the
existing 3.x series.

Therefore, we cannot simply remove support for the 3.x series, and
allow using only the 4.x series.

So what this commit does is extend the boot/at91bootstrap3 package to
support building both 3.x and 4.x versions. In detail, this implies:

 * Having the BR2_TARGET_AT91BOOTSTRAP3_LATEST_VERSION symbol point to
   the latest 4.x version. Indeed, we want
   BR2_TARGET_AT91BOOTSTRAP3_LATEST_VERSION to really point to the
   latest upstream version, even if that means potential breakage for
   users. Users who want to use a fixed version of at91bootstrap
   should anyway not be using
   BR2_TARGET_AT91BOOTSTRAP3_LATEST_VERSION.

 * Introduce BR2_TARGET_AT91BOOTSTRAP3_LATEST_VERSION_3X for users who
   would like to use the latest 3.x series.

 * Adjust the installation logic, as images to install are now in
   build/binaries/*.bin instead of binaries/*.bin. In order to not
   have to differentiate 3.x and 4.x, we simply use $(wildcard ...) to
   expand the list of files to install.

 * To make it clear that boot/at91bootstrap3 supports both 3.x and
   4.x, we also update the prompt of the package.

at911bootstrap does not carry a license file; so far we were using
main.c as the license file, as it carries the license blurb. Now that we
have a known alternate version, we would need a per-version hash for
that file. However, this is a bit too cumbersome to handle, so just drop
using main.c as the license file. When upstream introduces a proper
license file, we can revisit the situation.

Update the two defconfigs that were using the upstream 3.9.3 version;
all other defconfigs are using custom tarballs or custom git trees.

Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
[Thomas: while this patch is based on previous work by Eugen, it was
reworked quite significantly.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
[yann.morin.1998@free.fr:
  - drop main.c as license file, explain why
  - update the two defconfigs
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>