toolchain: introduce BR2_TOOLCHAIN_HAS_SHADOW_PASSWORDS

Unfortunately, the official Blackfin toolchains are built without the
shadow password support, so our default Busybox configuration fails to
build.

Therefore, we introduce a new hidden knob
BR2_TOOLCHAIN_HAS_SHADOW_PASSWORDS, which is set to yes for Buildroot
internal toolchain, for toolchains generated by the Crosstool-NG
backend, for Glibc external toolchains and for Uclibc custom external
toolchains. It is left unset by the Blackfin toolchain profile.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>