kbuild: doc: fix references to other documents

All the files in Documentation/kbuild/ were converted to reST.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

Documentation/kbuild/kbuild.rst

@@ -237,7 +237,7 @@ This is solely useful to speed up test compiles.
 KBUILD_EXTRA_SYMBOLS
 --------------------
 For modules that use symbols from other modules.
-See more details in modules.txt.
+See more details in modules.rst.
 
 ALLSOURCE_ARCHS
 ---------------

Documentation/kbuild/kconfig-macro-language.rst

@@ -44,7 +44,7 @@ intermediate::
             def_bool y
 
 Then, Kconfig moves onto the evaluation stage to resolve inter-symbol
-dependency as explained in kconfig-language.txt.
+dependency as explained in kconfig-language.rst.
 
 
 Variables

Documentation/kbuild/makefiles.rst

@@ -924,7 +924,7 @@ When kbuild executes, the following steps are followed (roughly):
 	$(KBUILD_AFLAGS_MODULE) is used to add arch-specific options that
 	are used for assembler.
 
-	From commandline AFLAGS_MODULE shall be used (see kbuild.txt).
+	From commandline AFLAGS_MODULE shall be used (see kbuild.rst).
 
     KBUILD_CFLAGS_KERNEL
 	$(CC) options specific for built-in
@@ -937,7 +937,7 @@ When kbuild executes, the following steps are followed (roughly):
 
 	$(KBUILD_CFLAGS_MODULE) is used to add arch-specific options that
 	are used for $(CC).
-	From commandline CFLAGS_MODULE shall be used (see kbuild.txt).
+	From commandline CFLAGS_MODULE shall be used (see kbuild.rst).
 
     KBUILD_LDFLAGS_MODULE
 	Options for $(LD) when linking modules
@@ -945,7 +945,7 @@ When kbuild executes, the following steps are followed (roughly):
 	$(KBUILD_LDFLAGS_MODULE) is used to add arch-specific options
 	used when linking modules. This is often a linker script.
 
-	From commandline LDFLAGS_MODULE shall be used (see kbuild.txt).
+	From commandline LDFLAGS_MODULE shall be used (see kbuild.rst).
 
     KBUILD_LDS