Add toolchain_var.mk in template

toolchain_var.mk defines various related to toolchain:
e.g. TARGET_CC  TARGET_ARCH

these variables will be used to generate toolchain wrapper.

Signed-off-by: Kui Shi <skuicloud@gmail.com>
Copy toolchain_var.mk to build project directory

Signed-off-by: Kui Shi <skuicloud@gmail.com>