lib/libc: picolibc: select its source with Kconfig choice

Implement the selection of the picolibc source in a Kconfig
choice as they are mutually exclusive.

The following table represents the possible choices, where 'X'
means that `PICOLIBC_SUPPORTED=n` and is guarded in the parent
level as `PICOLIBC=n`, so we do not need to care.

Module  Toolchain    C++   |       Choice
  0         0         0    |          X
  0         0         1    |          X
  0         1         0    |    Toolchain only
  0         1         1    |    Toolchain only
  1         0         0    |      Module only
  1         0         1    |          X
  1         1         0    |   Toolchain/Module
  1         1         1    |    Toolchain only

The current implementation favors `PICOLIBC_USE_TOOLCHAIN` over
`PICOLIBC_USE_MODULE` whenever possible, that preference is
maintained in this implementation as well - the TOOLCHAIN
source will be the default choice when the toolchain supports
it or when the C++ is enabled. Otherwise, fallback to MODULE.

Signed-off-by: Yong Cong Sin <[email protected]>

lib/libc/picolibc/Kconfig

@@ -3,9 +3,13 @@
 
 if PICOLIBC
 
+choice PICOLIBC_SOURCE
+	prompt "Source of Picolibc"
+	default PICOLIBC_USE_TOOLCHAIN if REQUIRES_FULL_LIBCPP || "$(TOOLCHAIN_HAS_PICOLIBC)" = "y"
+	default PICOLIBC_USE_MODULE
+
 config PICOLIBC_USE_MODULE
-	bool "Picolibc as module" if "$(TOOLCHAIN_HAS_PICOLIBC)" = "y"
-	default y if "$(TOOLCHAIN_HAS_PICOLIBC)" != "y"
+	bool "Picolibc from module"
 	depends on ZEPHYR_PICOLIBC_MODULE
 	depends on !GLIBCXX_LIBCPP
 	help
@@ -15,9 +19,14 @@ config PICOLIBC_USE_MODULE
 
 # force TLS when using toolchain with TLS support
 config PICOLIBC_USE_TOOLCHAIN
-	bool
-	default y if !PICOLIBC_USE_MODULE
+	bool "Picolibc from toolchain"
+	depends on "$(TOOLCHAIN_HAS_PICOLIBC)" = "y"
 	select THREAD_LOCAL_STORAGE if ARCH_HAS_THREAD_LOCAL_STORAGE && TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE
+	help
+	  Use picolibc included with the toolchain.
+	  This is required when using a full C++ standard library (`REQUIRES_FULL_LIBCPP=y`).
+
+endchoice # PICOLIBC_SOURCE
 
 choice PICOLIBC_IO_LEVEL
 	prompt "Picolibc printf/scanf level"