rust: Use variable to specify extra tools to install

All architectures may not support same set of tools, therefore use a
variable to specify this. E.g. on riscv32 rustfmt is not buildable
right now.

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>