sstate: allow specifying indirect dependencies to exclude from sysroot

sstate: allow specifying indirect dependencies to exclude from sysroot

Currently, a dependency on any -native recipe will pull in
all dependencies of that -native recipe in the recipe
sysroot. This behaviour might not always be wanted, e.g.
when that -native recipe depends on build-tools that are
not relevant for the current recipe.

This change adds a SSTATE_EXCLUDEDEPS_SYSROOT variable,
which will be evaluated for such recursive dependencies to
be excluded. The idea is similar to
   http://lists.openembedded.org/pipermail/openembedded-core/2018-January/146324.html
except that the list is not hard-coded anymore.

SSTATE_EXCLUDEDEPS_SYSROOT is evaluated as two regular
expressions of recipe and dependency to ignore, e.g. in
the above flex-native / bison-native use-case, one would
specify

    SSTATE_EXCLUDEDEPS_SYSROOT = ".*->(flex|bison)-native"

in layer.conf.

The existing special handling of "-initial" as well as
"base-passwd" and "shadow-sysroot" could also be
streamlined:

    SSTATE_EXCLUDEDEPS_SYSROOT += "\
        .*->.*-initial.* \
        .*(base-passwd|shadow-sysroot)->.* \
    "

Another anticipated user is meta-java, where certain newer
JDKs can only be bootstrapped (built) using older JDKs,
but it doesn't make much sense to copy all those older
JDKs and their own build tools (ant, etc.) into the
sysroot of recipes wanting to be built using the newer JDK
(only), e.g.:

    SSTATE_EXCLUDEDEPS_SYSROOT += "\
        openjdk-8-native->(ant-native|attr-native|coreutils-native|icedtea7-native|libxslt-native|make-native|openssl-native|zip-native|unzip-native) \
    "

Signed-off-by: André Draszik <andre.draszik@jci.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>