tools: ynl-gen: allow overriding name-prefix for constants

tools: ynl-gen: allow overriding name-prefix for constants

Allow using custom name-prefix with constants,
just like it is for enum and flags declarations.

This is needed for generating WG_KEY_LEN in
include/uapi/linux/wireguard.h from a spec.

Signed-off-by: Asbjørn Sloth Tønnesen <ast@fiberby.net>
Reviewed-by: Donald Hunter <donald.hunter@gmail.com>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250915144301.725949-2-ast@fiberby.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>