tools: Add 'firmware' category and add ihex2fw tool

Commit 5620a0d1 ("firmware: delete in-kernel firmware") removed
ihex2fw tool together with the rest of the contents of firmware/
folder. Since that tool is quite useful for doing .ihex -> .fw
converstion, restore its original source code to tools/firmware

Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Kyle McMartin <kyle@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-kernel <linux-kernel@vger.kernel.org>
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>