Correct logic for obtaining the path to full recovery image

Currently, an extra '/vendor' is appended in target_files_dir of
make_recovery_patch.py, which will yield an erroneous path when
attempting to build full recovery image on vendorimage-leas devices:

SYSTEM/vendor/vendor/etc/recovery.img

This patch addresses the issue by removing the extra '/vendor' of
target_files_dir, and add checks for whether the target builds
vendor image in MakeRecoveryPatch() as well. This ensures no
recovery image will be generated with prebuilt vendor.

Signed-off-by: Ricky Cheung <rcheung844@gmail.com>
Change-Id: I2dc6e43537deb606dd01fb090add2595502055c1