Which programming language has very short context-free Grammar in its formal specification?

I haven't compared, but Lua is a language renowned for its simple syntax. The BNF is at the very end of this reference manual: http://www.lua.org/manual/5.1/manual.html .

One of the simplest imperative languages is Oberon-2. Syntax of Oberon-2.

Also take a look at Oberon-07 (The Programming Language Oberon-07, PDF) and Component Pascal.

Assembly languages!

...in general, and particularly for CPUs which have a simple architecture (few instructions, few addressing modes, few registers) have a relatively short grammar.

In fact, specialized processors, such as these found in programmable logic controllers can have a language with even simpler grammars. But then again the most simple of the PLCs are little more than Boolean equation calculators.