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

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

Commit 654102df2ac2 ("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_BUILTIN_DTB_SOURCE to CONFIG_BUILTIN_DTB_NAME.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20241222000836.2578171-1-masahiroy@kernel.org
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>