BaseTools: Fixed the mis-using strip() function issue.

BaseTools: Fixed the mis-using strip() function issue.

BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2003

lstrip(parameter) do the match based on the char
in parameter but not only the whole parameter string.

In GenMake line 1082,
CmdSign.lstrip('/Fo') will strip the '/' or
'F' or 'o' on the left of CmdSign. This is not expected.

This patch is going to fix such issue.

Cc: Liming Gao <liming.gao@intel.com>
Signed-off-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>