*config: improve handling of BR2_LEGACY

In 8a46d4bf the randpackageconfig and allpackageyesconfig were
extended with disabling all the legacy options, otherwise the resulting
config couldn't be built. However, that didn't work for randconfig and
allyesconfig.

This commit reverts 8a46d4bf and replaces it with a different
approach: skipping of the legacy config options is passed explicitly
through the environment variable SKIP_LEGACY, which forces
BR2_SKIP_LEGACY to y. The new option BR2_SKIP_LEGACY completely
disables the legacy handling, which effectively removes all the legacy
options from the .config.

However, in that case no values are set for the legacy options so a
subsequent oldconfig will query them. Therefore, run an additional
olddefconfig.

[Peter: fix s/BR2_LEGACY/SKIP_LEGACY/ typo]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[yann.morin.1998@free.fr: do not rely on a user-visible option, works
 perfectly well with only blind options set from the environment]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>