genrandconfig: use minimal.config

genrandconfig: use minimal.config

This has a number of side-effects which must be handled.

The lines in minimal.config may be overridden by the random lines added
by amending the configuration, so is_toolchain_usable() shouldn't take
those into account, or indeed the random lines added. Therefore, make
a copy of the config before appending minimal.config and the random
lines. While we're at it, rename the variable to the more appropriate
toolchainconfig.

minimal.config sets BR2_INIT_NONE=y, but we really also want to test
with BR2_INIT_BUSYBOX=y. Therefore, add a random line to use the
busybox init system. We set its probability rather high. The
probabilities of systemd and eudev are increased since they're now
in the else branch of BR2_INIT_BUSYBOX, which halves the probability
that we even get there.

We now also generate configurations without busybox. Previously,
busybox was almost always selected due to BR2_INIT_BUSYBOX=y. Only if
systemd is selected there was a chance to build without busybox.

We now set BR2_SYSTEM_BIN_SH_NONE=y, the other /bin/sh options are
never tested. However, this is not really something that is relevant
to test in the autobuilders.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>