configs/acmesystems_acqua_a5_*: use tested bootloader version

The new branch "4.x" of at91bootstrap has dropped the contrib directory,
which holds the defconfigs used by the Acmesystems Acqua A5 boards. We
then cannot use the latest at91bootstrap anymore.

As commit e009816c introduced support
for at91bootstrap 4.x in the "next" branch, it had to work around this
by pinning these defconfigs to the "latest 3.x" version.

Avoid this and any future incompatibility problems by explicitly
requesting a tested version of at91bootstrap.

Signed-off-by: Edgar Bonet <bonet@grenoble.cnrs.fr>
[yann.morin.1998@free.fr: use a custom git, like other acme defconfigs]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>