buildroot

Makefile: remove 'toolchain' from .PHONY

toolchain is a package, so it is already defined as .PHONY in the
inner-generic-package macro.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>