kbuild: make V=2 tell why a target is rebuild

tell why a a target got build
   enabled by make V=2
      Output (listed in the order they are checked):
         (1) - due to target is PHONY
         (2) - due to target missing
         (3) - due to: file1.h file2.h
         (4) - due to command line change
         (5) - due to missing .cmd file
         (6) - due to target not in $(targets)
(1) We always build PHONY targets
(2) No target, so we better build it
(3) Prerequisite is newer than target
(4) The command line stored in the file named dir/.target.cmd
    differed from actual command line. This happens when compiler
    options changes
(5) No dir/.target.cmd file (used to store command line)
(6) No dir/.target.cmd file and target not listed in $(targets)
    This is a good hint that there is a bug in the kbuild file

This patch is inspired by a patch from: Milton Miller <miltonm@bga.com>

Cc: Milton Miller <miltonm@bga.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

1 files changed, 1 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index 22451c2ac1e0..440a1331d883 100644
--- a/Makefile
+++ b/Makefile
@@ -1101,6 +1101,7 @@ help:
                 echo '')
 
         @echo  '  make V=0|1 [targets] 0 => quiet build (default), 1 => verbose build'
+        @echo  '  make V=2   [targets] 2 => give reason for rebuild of target'
         @echo  '  make O=dir [targets] Locate all output files in "dir", including .config'
         @echo  '  make C=1   [targets] Check all c source with $$CHECK (sparse by default)'
         @echo  '  make C=2   [targets] Force check of all c source with $$CHECK'