scripts: make rust-analyzer for out-of-tree modules

Adds support for out-of-tree rust modules to use the `rust-analyzer`
make target to generate the rust-project.json file.

The change involves adding an optional parameter `external_src` to the
`generate_rust_analyzer.py` which expects the path to the out-of-tree
module's source directory. When this parameter is passed, I have chosen
not to add the non-core modules (samples and drivers) into the result
since these are not expected to be used in third party modules. Related
changes are also made to the Makefile and rust/Makefile allowing the
`rust-analyzer` target to be used for out-of-tree modules as well.

Link: https://github.com/Rust-for-Linux/linux/pull/914
Link: https://github.com/Rust-for-Linux/rust-out-of-tree-module/pull/2
Signed-off-by: Vinay Varma <varmavinaym@gmail.com>
Link: https://lore.kernel.org/r/20230411091714.130525-1-varmavinaym@gmail.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Makefile

@@ -1859,11 +1859,6 @@ rustfmt:
 rustfmtcheck: rustfmt_flags = --check
 rustfmtcheck: rustfmt
 
-# IDE support targets
-PHONY += rust-analyzer
-rust-analyzer:
-	$(Q)$(MAKE) $(build)=rust $@
-
 # Misc
 # ---------------------------------------------------------------------------
 
@@ -1924,6 +1919,7 @@ help:
 	@echo  '  modules         - default target, build the module(s)'
 	@echo  '  modules_install - install the module'
 	@echo  '  clean           - remove generated files in module directory only'
+	@echo  '  rust-analyzer	  - generate rust-project.json rust-analyzer support file'
 	@echo  ''
 
 __external_modules_error:
@@ -2065,6 +2061,11 @@ quiet_cmd_tags = GEN     $@
 tags TAGS cscope gtags: FORCE
 	$(call cmd,tags)
 
+# IDE support targets
+PHONY += rust-analyzer
+rust-analyzer:
+	$(Q)$(MAKE) $(build)=rust $@
+
 # Script to generate missing namespace dependencies
 # ---------------------------------------------------------------------------