Press and hold Volume Down, then press and hold Power
adb reboot-bootloader
fastboot oem unlock

. build/envsetup.sh
lunch aosp_bullhead-userdebug

set ANDROID_PRODUCT_OUT=W:\home\sonic\marshmallow\out\target\product\bullhead
fastboot -w flashall

adb disable-verity
adb reboot
adb root
adb remount
adb shell
echo "ro.vendor.build.date=$(getprop ro.build.date)" > /vendor/build.prop
echo "ro.vendor.build.date.utc=$(getprop ro.build.date.utc)" >> /vendor/build.prop
echo "ro.vendor.build.fingerprint=$(getprop ro.build.fingerprint)" >> /vendor/build.prop
sync;sync;reboot

root@bullhead:/ # ls -l /system/xbin/su
-rwsr-x--- root     shell       10176 2015-12-15 01:14 su


https://code.google.com/p/aosp4nexus5/wiki/AOSPforNexus5#Add_support_for_GApps_(_Google_Play_Store_)