icu: not available when BR2_BINFMT_FLAT is used

icu does not recognize 'uclinux' as a supported platform. While making
it recognize uclinux is easy, there is another problem down the road:
icu does very weird things to generate an ELF library containing
static data (libicudata.a), and the generated library being ELF, it is
not compatible with the FLAT binary format expected by uclinux
platforms such as Blackfin in FLAT format.

Therefore, we simply disallow the selection of icu on FLAT
platforms.

Note that adding a dependency on BR2_BINFMT_ELF doesn't work, because
BR2_BINFMT_FDPIC is considered to be separate (even if technically
FDPIC is a derivative of ELF). That's why the dependency we're adding
is "depends on !BR2_BINFMT_FLAT" and not "depends on BR2_BINFMT_ELF".

Fixes:

  http://autobuild.buildroot.org/results/b41/b415fed7fae4012bad7d8b53a481bd71bdab716f/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>