In the next commit, I need the image name in lowercase because
output files are generally named in lowercase.

Unfortunately, TOOL_ADD_IMG takes the first argument in uppercase
since we generally use uppercase Make variables.

make_helpers/build_macros.mk provides 'uppercase' macro to convert
a string into uppercase, but 'lowercase' does not exist.  We can
implement it if we like, but it would be more straightforward to
change the argument of TOOL_ADD_IMG.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>