toaster: hide special code for core vars not present, disable add button

toaster: hide special code for core vars not present, disable add button

Hide the special config variable management sections when the respective
variables are not present. Disable the variable add button after a
variable is added so that blank values cannot be added, and also when a
variable name managed in the project detail page (e.g. BBLAYERS and MACHINE)
is entered.

[YOCTO #7246]
[YOCTO #7371]

Signed-off-by: David Reyna <David.Reyna@windriver.com>