Generate def files in $(genDir) instead of inside Android source tree

Android source tree will eventually be made ReadOnly with /tmp and /out
as ReadWrite directories
This CL will ensure that the intermediate def files will be written to
the sandbox directory $(genDir) instead of $TOP

Bug: 174726238
Test: Built a target in git_master in a RO source tree
1. Before CL, we get this error "bash: i386_defs: Read-only file system"
2. After CL, the above error is fixed

Change-Id: I06c7e472efee8641d43b0ae8efca9323afca916b
1 file changed