jquery-ui-themes: rename options to have proper prefix

The choice options to select the specific jquery-ui theme to install
had a prefix of BR2_PACKAGE_JQUERY_UI_THEME_ instead of
BR2_PACKAGE_JQUERY_UI_THEMES_. This commit fixes that, and adds
Config.in.legacy handling. It's worth mentioning that since those
options are part of a choice, the legacy handling cannot select the
new name of the option: we can simply inform the user about the
renaming.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>