docs: kbuild: add documentation for some Rust variables

    RUSTFLAGS_KERNEL
    RUSTFLAGS_MODULE
    KBUILD_RUSTFLAGS
    KBUILD_RUSTFLAGS_KERNEL
    KBUILD_RUSTFLAGS_MODULE

Furthermore, move `KRUSTFLAGS` a bit up to be closer to `KCFLAGS`
since all the `K*FLAGS` seem to go together (rather than per-language).

Signed-off-by: Miguel Ojeda <[email protected]>

Author: ojeda

Documentation/kbuild/kbuild.rst

@@ -48,6 +48,10 @@ KCFLAGS
 -------
 Additional options to the C compiler (for built-in and modules).
 
+KRUSTFLAGS
+----------
+Additional options to the Rust compiler (for built-in and modules).
+
 CFLAGS_KERNEL
 -------------
 Additional options for $(CC) when used to compile
@@ -57,9 +61,14 @@ CFLAGS_MODULE
 -------------
 Additional module specific options to use for $(CC).
 
-KRUSTFLAGS
-----------
-Additional options to the Rust compiler (for built-in and modules).
+RUSTFLAGS_KERNEL
+----------------
+Additional options for $(RUSTC) when used to compile
+code that is compiled as built-in.
+
+RUSTFLAGS_MODULE
+----------------
+Additional module specific options to use for $(RUSTC).
 
 LDFLAGS_MODULE
 --------------

Documentation/kbuild/makefiles.rst

@@ -1179,6 +1179,14 @@ When kbuild executes, the following steps are followed (roughly):
 	The first example utilises the trick that a config option expands
 	to 'y' when selected.
 
+    KBUILD_RUSTFLAGS
+	$(RUSTC) compiler flags
+
+	Default value - see top level Makefile
+	Append or modify as required per architecture.
+
+	Often, the KBUILD_RUSTFLAGS variable depends on the configuration.
+
     KBUILD_AFLAGS_KERNEL
 	Assembler options specific for built-in
 
@@ -1206,6 +1214,19 @@ When kbuild executes, the following steps are followed (roughly):
 	are used for $(CC).
 	From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
 
+    KBUILD_RUSTFLAGS_KERNEL
+	$(RUSTC) options specific for built-in
+
+	$(KBUILD_RUSTFLAGS_KERNEL) contains extra Rust compiler flags used to
+	compile resident kernel code.
+
+    KBUILD_RUSTFLAGS_MODULE
+	Options for $(RUSTC) when building modules
+
+	$(KBUILD_RUSTFLAGS_MODULE) is used to add arch-specific options that
+	are used for $(RUSTC).
+	From commandline RUSTFLAGS_MODULE shall be used (see kbuild.rst).
+
     KBUILD_LDFLAGS_MODULE
 	Options for $(LD) when linking modules