emulator: add check_emu_boot.mk

"make check_emu_boot" will boot up emulator
and check whether it boots up or timed out.
On boot success, it will emit a file
BOOT_SUCCESS.txt in dist_dir;
On timed out, it will emit a file
BOOT_FAIL.txt in dist_dir.

Change-Id: I152228806175c116a5adceb8429b66cf829edd22