Add the `LIBC_PREVENT_STRING_KERNEL` and `LIBC_PREVENT_STRING_USER`
that are meant to be selected by the chip if no libc implementation
is going to be built. If selected, neither NuttX's software version
of the libc nor any architecture-specific implementation will be
built in the kernel or in the userspace, respectively. In this
case, the linker may provide a ROM-defined version of the libc
functions instead.
Do not call strlen() here. Old implementation iterated
over string twice, if searched for position was at the
beginning. This commit changes strrchr() to scan string
only once, regardless of input.
Signed-off-by: Juha Niskanen <juha.niskanen@haltian.com>
