nios2: migrate to the generic rule for built-in DTB

Commit 654102df ("kbuild: add generic support for built-in boot
DTBs") introduced generic support for built-in DTBs.

Select GENERIC_BUILTIN_DTB when built-in DTB support is enabled.

To keep consistency across architectures, this commit also renames
CONFIG_NIOS2_DTB_SOURCE_BOOL to CONFIG_BUILTIN_DTB, and
CONFIG_NIOS2_DTB_SOURCE to CONFIG_BUILTIN_DTB_NAME.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>