mysql: propagate common dependencies to toplevel config

mysql: propagate common dependencies to toplevel config

Commit 3d707d2b (mysql: rename package to oracle-mysql, make a virtual
package) introduced a user selectable virtual BR2_PACKAGE_MYSQL package, but
didn't propagate the (common) dependencies of the two variants to it, so the
virtual package can now be selected even though neither of the variants are
available.

As several packages enable mysql support when BR2_PACKAGE_MYSQL is selected,
this causes a number of autobuilder issues:

http://autobuild.buildroot.net/results/7fe/7fe0d0a3e7ed0430852dc42b718dd037557207e8/
http://autobuild.buildroot.net/results/cc4/cc4c2d936f3e1ba6c0a9782b2218de54a4ff75d2/

Fix it by propagating the common dependencies of the two variants to the
virtual package to ensure it cannot be enabled unless at least one of them
are available.

Also move the toolchain comment outside the conditional so it is visible
when mysql isn't available.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>