libc: Default to picolibc where supported

This switches the default C library to picolibc for all targets which
support it.

Signed-off-by: Keith Packard <keithp@keithp.com>

lib/libc/Kconfig

@@ -55,8 +55,8 @@ menu "C Library"
 choice LIBC_IMPLEMENTATION
  prompt "C Library Implementation"
  default EXTERNAL_LIBC if NATIVE_APPLICATION
+ default PICOLIBC
  default NEWLIB_LIBC if REQUIRES_FULL_LIBC
- default PICOLIBC if REQUIRES_FULL_LIBC
  default MINIMAL_LIBC
 
 config MINIMAL_LIBC