kbuild: consolidate version and timestamp headers generation

- Generate include/generated/{timestamp.h, version.h}
   more simply by using filechk rule.

 - Add $(UBOOTRELEASE) variable and re-write u-boot.imx rule
   more simply.

 - Rename U_BOOT_VERSION in Makefile to UBOOTVERSION

   Before this commit, the same variable name, "U_BOOT_VERSION"
   was used for two different strings.

   One of them was defined in Makefile.
   It takes the form like "2014.01-rc1" and used in
   makefiles and script files.

   The other is defined in include/generated/version.h
   It takes the form like "U-Boot 2014.01-rc1-00010-gbe6d426-dirty"
   and used in C and Aseembler.

   It is confusing when grepping the source tree. So, this commit
   renames the former to UBOOTVERSION.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>

doc/DocBook/Makefile

@@ -131,7 +131,7 @@ index = index.html
 main_idx = doc/DocBook/$(index)
 build_main_index = rm -rf $(main_idx); \
 		   echo '<h1>U-Boot Bootloader HTML Documentation</h1>' >> $(main_idx) && \
-		   echo '<h2>U-Boot Version: $(U_BOOT_VERSION)</h2>' >> $(main_idx) && \
+		   echo '<h2>U-Boot Version: $(UBOOTVERSION)</h2>' >> $(main_idx) && \
 		   cat $(HTML) >> $(main_idx)
 
 quiet_cmd_db2html = HTML    $@