kbuild: document KBUILD_LDS, KBUILD_VMLINUX_{INIT,MAIN} and LDFLAGS_vmlinux

Newly exported variables - used by link-vmlinux.sh
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>

Documentation/kbuild/kbuild.txt

@@ -50,6 +50,10 @@ LDFLAGS_MODULE
 --------------------------------------------------
 Additional options used for $(LD) when linking modules.
 
+LDFLAGS_vmlinux
+--------------------------------------------------
+Additional options passed to final link of vmlinux.
+
 KBUILD_VERBOSE
 --------------------------------------------------
 Set the kbuild verbosity. Can be assigned same values as "V=...".
@@ -214,3 +218,18 @@ KBUILD_BUILD_USER, KBUILD_BUILD_HOST
 These two variables allow to override the user@host string displayed during
 boot and in /proc/version. The default value is the output of the commands
 whoami and host, respectively.
+
+KBUILD_LDS
+--------------------------------------------------
+The linker script with full path. Assigned by the top-level Makefile.
+
+KBUILD_VMLINUX_INIT
+--------------------------------------------------
+All object files for the init (first) part of vmlinux.
+Files specified with KBUILD_VMLINUX_INIT are linked first.
+
+KBUILD_VMLINUX_MAIN
+--------------------------------------------------
+All object files for the main part of vmlinux.
+KBUILD_VMLINUX_INIT and KBUILD_VMLINUX_MAIN together specify
+all the object files used to link vmlinux.