toolchain: move glibc nsswitch.conf handling to a post-target hook

toolchain: move glibc nsswitch.conf handling to a post-target hook

nsswitch.conf is processed both by the toolchain, nss-mdns and
nss-myhostname without any guaranteed ordering in between.

The toolchain package ensures that nsswitch.conf is available, and the two
nss-* packages tweaks the content, so the toolchain processing should run
before the nss-* ones.  Toolchain is a dependency of all the packages, so
ensure this is done by moving the toolchain handling to a
post-target-install hook.

Also move the variable to toolchain/toolchain/toolchain.mk where the virtual
toolchain package is defined for clarity.

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