at91bootstrap3: make sure AT91BOOTSTRAP3_KCONFIG_FILE is always defined

For some (silly?) reason, the kconfig-package infra always wants
<pkg>_KCONFIG_FILE to be defined. So let's do like barebox does: use a
temporary AT91BOOTSTRAP3_SOURCE_CONFIG variable, and assign it to
AT91BOOTSTRAP3_KCONFIG_FILE. This way, the latter is always defined,
even if to the empty value.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>