package.bbclass: exclude packages as shlib providers
Some packages containing shared libraries might be registered
as shlib providers when they shouldn't (for example, the lib is for
their private use and must not generate any dependency).
EXCLUDE_FROM_SHLIBS is targeted at that, but it could be set
for entire recipe only.
This patch allows to set list of packages that are not candidates
for shlib providers.
Variable EXCLUDE_PACKAGES_FROM_SHLIBS is used, e.g.:
EXCLUDE_PACKAGES_FROM_SHLIBS = "glibc-ptest glibc-mtest"
