mtd-utils: add mtd test tools

mtd-utils: add mtd test tools

Add the possibility to choose more test tools than only integck
from the MTD utils test-suite.

Move the hidden BR2_PACKAGE_MTD_TESTS configuration to the bottom
of the page and make it visible. When checked, a new list of available
binaries is displayed and may be selected and compiled into the image
with the --enable-install-tests configure script option:
- flash_torture
- flash_stress
- flash_speed
- nandbiterrs
- flash_readtest
- nandpagetest
- nandsubpagetest

Most of these tests may be performed by inserting kernel modules
which are almost legacy so having the userspace tools available
might become useful.

Legacy handling for users who had BR2_PACKAGE_MTD_INTEGCK selected is
not needed: they also have the BR2_PACKAGE_MTD_TESTS option enabled in
their .config. That option has now become a selectable option so it is
not removed any more by oldconfig.

Signed-off-by: Miquel Raynal <miquel.raynal@free-electrons.com>
[Arnout:
 - fix threads and MMU dependency;
 - order Config.in options and mtd.mk lines alphabetically;
 - add note on legacy handling to commit message.]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
fixup