Enable uploading ramdisk to target img.zip

For some devices, the img.zip package needs to contain the ramdisk
in order to enable device-specific flashing to work properly

This change adds a new flag BOARD_IMG_USE_RAMDISK to the possible
device Makefile configuration. If set to true, the build process
will insert ramdisk.img in the target-files.zip and img.zip
build outputs of "m dist". No change will occur for builds
that do not use this flag, or set it to a non-true value.

Bug: 168642807
Test: lunch trout_arm64-userdebug, m dist with and w/out
      BOARD_IMG_USE_RAMDISK added to Makefile;
      lunch aosp_cf_arm64-phone-userdebug, m dist
Change-Id: Id29408551cd41c11b96157248e238324a327043d
Merged-In: Id29408551cd41c11b96157248e238324a327043d