buildroot: fix lisa-buildroot-update-kernel-config

unset CROSS_COMPILE to avoid Kconfig detecting GCC features, which will
make it ask this kind of questions later when running `make Image` if
the CROSS_COMPILE toolchain is different at that point:
GCC plugins (GCC_PLUGINS) [N/y/?] (NEW)

Restore auto-detection of ARCH, since we can autodetect all required env
var.

Also use KConfig fragments, which allows splitting config file update
from fixup of values along dependencies. As a bonus, KConfig fragment
reports what was modified in the config (or at least parts of what was
modified).