BaseTools/Ecc: Fix an issue of path separator compatibility

BaseTools/Ecc: Fix an issue of path separator compatibility

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2904

The path separator is different in Windows and Linux, the
original code does not handle this difference. This patch
is to fix this issue.

Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Yuwei Chen <yuwei.chen@intel.com>
Cc: Shenglei Zhang <shenglei.zhang@intel.com>
Message-Id: <20200901102315.38840-1-bob.c.feng@intel.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>