make: add defconfig, don't run silentoldconfig before menuconfig

make: add defconfig, don't run silentoldconfig before menuconfig

This commit fixes the case when new Kconfig configuration options were added and you run `make menuconfig`.
Previously this would first show console prompt to set values for new options, and then launch menuconfig.
Now this will just launch menuconfig.
New target is added, defconfig, which may be useful for non-interactive environments to set default values for
all new configuration options.