bitbake: toaster: do not create duplicate HelpText objects

bitbake: toaster: do not create duplicate HelpText objects

buildinfohelper code expects only one HelpText object per
build/variable/description.
Current code creates more than one such an object, which causes
toastergui to crash with this exception:
 MultipleObjectsReturned: get() returned more than one HelpText -- it returned 2!

Used git_or_create API to ensure that only one HelpText object is
created.

(Bitbake rev: e9b46803eb6f1f4044919abf90c8aeb3536e73ed)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>