libcutils: fs_config: target_out_path assumed /system breaking partitions

Strip off trailing / then /system and then add back the appropriate
config directory.  This fixes an issue with reading vendor, oem or odm
partitions.

Test: manual build successfully interprets all etc/fs_config_* files.
Test: manual incremental build successfully interprets all etc/fs_config_* files.
Bug: 36071012
Change-Id: Iba363f0731bb8d15e595bb45c56db97722edabc2