Make bootstrap.bat more portable

Make bootstrap.bat more portable

Using backslashes in a sed command line is tricky as different programs
use them differently as escape sequences.  Eli Zaretskii points out
that Windows "echo" doesn't do any processing, so rework all our sed
invocations to use script files created by echo.

* bootstrap.bat: Use echo to create sed script files instead of -e.
* Basic.mk.template: Fix typo in the comments.
* .gitignore: Ignore any .sed scripts.