utils/genrandconfig: add mxs-bootlets board handling

Add a custom case to make sure that a random configuration with an empty
board for mxs-bootlets doesn't fail. It reverts to
BR2_TARGET_MXS_BOOTLETS_STMP37xx in that case.

>>> mxs-bootlets 10.12.01 Building
BOARD= CROSS_COMPILE="/home/thomas/autobuild/instance-1/output-1/per-package/mxs-bootlets/host/bin/arm-buildroot-linux-uclibcgnueabi-" /usr/bin/make -j1 -C /home/thomas/autobuild/instance-1/output-1/build/mxs-bootlets-10.12.01 power_prep
/home/thomas/autobuild/instance-1/output-1/per-package/xinetd/host/bin/arm-buildroot-linux-uclibcgnueabi-gcc -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -O2 -g0  -static -DNO_RPC  -I../../include   -c -o inet_aton.o inet_aton.c
make[1]: Entering directory '/home/thomas/autobuild/instance-1/buildroot'
make[1]: warning: -j1 forced in submake: resetting jobserver mode.
build power_prep
/usr/bin/make -C power_prep ARCH= BOARD=
make[2]: Entering directory '/home/thomas/autobuild/instance-1/output-1/build/mxs-bootlets-10.12.01/power_prep'
/usr/bin/make -C ./../mach-/hw
make[3]: Entering directory '/home/thomas/autobuild/instance-1/output-1/build/mxs-bootlets-10.12.01/power_prep'
make[3]: *** ../mach-/hw: No such file or directory.  Stop.

Fixes:
 - http://autobuild.buildroot.org/results/44a2efc64b9b8ff4541430d6b649e7a11a4e4873



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>