libc/picolibc: Only enable malloc when required

When malloc is included in the build, the heap initialization code will be
pulled in even if the application doesn't actually use malloc. This
increases the size of executables by a few hundred bytes and delays system
startup.

Select COMMON_LIBC_MALLOC only when applications indicate that they require
malloc by setting REQUIRES_MALLOC.

Signed-off-by: Keith Packard <[email protected]>

lib/libc/Kconfig

@@ -89,7 +89,7 @@ config PICOLIBC
 	select COMMON_LIBC_ABORT
 	select THREAD_LOCAL_STORAGE if ARCH_HAS_THREAD_LOCAL_STORAGE && TOOLCHAIN_SUPPORTS_THREAD_LOCAL_STORAGE
 	select LIBC_ERRNO if THREAD_LOCAL_STORAGE
-	imply COMMON_LIBC_MALLOC
+	select COMMON_LIBC_MALLOC if REQUIRES_MALLOC
 	depends on !NATIVE_APPLICATION
 	depends on PICOLIBC_SUPPORTED
 	help