kbuild: add generic support for built-in boot DTBs

Some architectures embed boot DTBs in vmlinux. A potential issue for
these architectures is a race condition during parallel builds because
Kbuild descends into arch/*/boot/dts/ twice.

One build thread is initiated by the 'dtbs' target, which is a
prerequisite of the 'all' target in the top-level Makefile:

ifdef CONFIG_OF_EARLY_FLATTREE
all: dtbs
endif

For architectures that support the built-in boot dtb, arch/*/boot/dts/
is visited also during the ordinary directory traversal in order to
build obj-y objects that wrap DTBs.

Since these build threads are unaware of each other, they can run
simultaneously during parallel builds.

This commit introduces a generic build rule to scripts/Makefile.vmlinux
to support embedded boot DTBs in a race-free way. Architectures that
want to use this rule need to select CONFIG_GENERIC_BUILTIN_DTB.

After the migration, Makefiles under arch/*/boot/dts/ will be visited
only once to build only *.dtb files.

This change also aims to unify the CONFIG options used for built-in DTBs
support. Currently, different architectures use different CONFIG options
for the same purposes.

With this commit, the CONFIG options will be unified as follows:

- CONFIG_GENERIC_BUILTIN_DTB

This enables the generic rule for built-in boot DTBs. This will be
renamed to CONFIG_BUILTIN_DTB after all architectures migrate to the
generic rule.

- CONFIG_BUILTIN_DTB_NAME

This specifies the path to the embedded DTB.
(relative to arch/*/boot/dts/)

- CONFIG_BUILTIN_DTB_ALL

If this is enabled, all DTB files compiled under arch/*/boot/dts/ are
embedded into vmlinux. Only used by MIPS.

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