scrips/kconfig: use snprintf() vs sprintf()

Coverity reported 150819 issue, which steams off Flex generated code
from zconf.l in which sprintf() was use. Because of that, the
conf_read_simple() @name parameter could be used to overrun
zconf_open() @fullname by crafting SRCTREE and KCONFIG_ALLCONFIG
environment variables.

Change-Id: I2cff817dccafe0e06b35636bbb7be95e062410af
Coverity-ID: 150819
Signed-off-by: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>