Include merge_target_files.py deps in otatools.zip, use common argv processing.

The merge_target_files.py script needs fc_sort and sefcontext_compile, so
include these tools into otatools.zip via core/Makefile.

Modify tools/releasetools/merge_target_files.py to use the otatools common argv
processing to take advantage of the '--path' option so that we add point the
'--path' option to an extracted otatools.zip package to gain access to fc_sort,
sefcontext_compile, and soong_zip (previously included in otatools.zip).

Bug: 123600124
Test: extract otatools.zip, use --path option to point to it, verify result
Change-Id: I7d84525981b8741c6bdbcac9984256920fc7f417