vboot-utils: MTD_CHAR_MAJOR definition

Fixes
http://autobuild.buildroot.net/results/a6360fd21314899321071c34fcb4892c5eed9bcc/
http://autobuild.buildroot.net/results/0af3aa4a429c45d3a2609913ec1040f32378f3c8/
http://autobuild.buildroot.net/results/cc4940fd6f51b31d71014f5aedee5223180eb36e/



Signed-off-by: Alex Suykov <alex.suykov@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>