Mizar32: Make inclusion of term.*() Lua module depend on BUILD_TERM

Just a config dependency change so that undefining BUILD_TERM
automatically disables the "term" module.
Allow BUILD_SHELL without BUILD_TERM

The only thing the shell uses from "term" is its getchar()
substitute, which ends up in platform_uart_getchar() on every
platform except i386. So if TERM is present, use its getchar()
redirector, and if it isn't, use platform_uart() directly.