linux: Add uImage support for MIPS architecture

uImage support for MIPS was added in the kernel on 2013-09-03, so we
allow to build uImage for MIPS in Buildroot.

Kernel commit:
  http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=4defe4559e86e26545ab2f542656a4b966dcde53



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>