esw.bbclass: Change 'or' to 'and' to verify EXTERNALSRC is defined
esw.bbclass: Change 'or' to 'and' to verify EXTERNALSRC is defined

Issue: CR-1059154

If the externalsrc is enabled, but EXTERNALSRC is not defined this is
and invalid configuration.  (The externalsrc class already looks for
this and errors.)

Doing this will ensure that EXTERNALSRC is always defined and we'll
avoid any errors related to 'None' values.

Signed-off-by: Mark Hatle <41765ab18fb75e64ff0efe27f2ebcdb2d6f7e44e@xilinx.com>

Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx,Xilinx/meta-xilinx