vndk-def-tool: Update README.md

This commit updates readme instructions to match with the simplified
VNDK in O workflow.

Bug: 37867089
Test: Run the described command lines with sailfish directories.
Change-Id: Ie8507fdbcdb1e83f56b465d2d5fce88129f817aa
Merged-In: I80f3ee4eac34fae772ba6c336e457f537cb8312c