u-boot: fix externalsrc not triggering do_configure on defconfig changes

u-boot: fix externalsrc not triggering do_configure on defconfig changes

externalsrc only monitors files listed in CONFIGURE_FILES environment
variable to know if it should trigger a rebuild of do_configure. By
default it is unset, but the defconfig from U-Boot should be listed
otherwise an old defconfig may be used even though the change is
technically detected by the do_compile logic later in the process.

Because U-Boot recipe uses `make oldconfig` when no defconfig is passed,
monitor .config for that special case.

This fixes U-Boot recipes not detecting defconfig changes when
devtool'ed.

Reported-by: Iskander Amara <iskander.amara@theobroma-systems.com>
Cc: Quentin Schulz <foss+yocto@0leil.net>
Signed-off-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>