BaseTools: fix the split output files root dir

BaseTools: fix the split output files root dir

If the output file path is a relative path, the split
tool will create the output file under the input file path.
But the expected behavior for this case is the output file
should be relative to the current directory. This patch will
fix this bug.

If the output file path is not specified and output prefix is not
specified, the output file should be under the input file path

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Acked-by: Liming Gao <gaoliming@byosoft.com.cn>
Reviewed-by: Yuwei Chen <yuwei.chen@intel.com>