Deal with "sed -i" portability

An empty extension for sed -i is not widely available.

* Where it isn't available (eg. macOS's sed) "sed -i -e" will
  create a backup file with the "-e" suffix.

* Even GNU sed documentation says it's "not recommended".

This commit deals with it by:

* Replace it with a more appropriate tool (kconfig-tweak)

* Or, specify the extension (.bak)

.gitignore

@@ -27,7 +27,6 @@
 /.clean_context
 /.config
 /.config.old
-/.config-e
 /.cproject
 /.gdbinit
 /.project
@@ -35,7 +34,6 @@
 /_SAVED_APPS_config
 /cscope.*
 /defconfig
-/defconfig.tmp-e
 /Make.defs
 /nuttx
 /nuttx-*