package/elf2flt: hide when an MMU is used

elf2flt is specifically for cases where an MMU is not available.
There is no point in building elf2flt when using an MMU.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>