efibootmgr: use the new gettext logic

efibootmgr: use the new gettext logic

This commit switches to use the new gettext logic, which involves:

 - using TARGET_NLS_DEPENDENCIES instead of hand-encoded dependencies
   on gettext/host-gettext

 - using TARGET_NLS_LIBS to force linking against libintl

 - dropping BR2_PACKAGE_GETTEXT selection

The BR2_USE_WCHAR dependency was needed for gettext, now that we no
longer need gettext, we can also get rid of this dependency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>