[amber_ctl] Discourage enabling multiple sources

When adding an enabled source or enabling a specific source, disable all
other enabled sources to prevent amber from picking one at random during
package installs or OTA updates.  This change also adds a "-x" flag to
disable this feature, for those that like to live dangerously.

PKG-382 #done

Test: manual, add enabled/disabled sources, enable sources, with and
without -x flag

Change-Id: I1554230a3e19c9ecbe960c5f2a7e2a88de71f36d
1 file changed