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>