samples: static_lib: conditional assign BOARD (?=)

The sample app "static_lib" is very important to Zephyr user, which
demonstrate how to build and link a static lib.
ISSM team wanted to integrate this app in their IDE for quark platforms.
However they find the in "static_lib/hello_world/Makefile" BOARD is
hardcoded as qemu_x86.

This patch supports other BOARD passed from build command.
I have verified this app working fine @Arduino101.

Signed-off-by: Sharron LIU <sharron.liu@intel.com>