rework thread handling

In the next uClibc-ng release Linuxthreads support will be
again allowed for architectures supporting NPTL.
To keep Linuxthreads supported and shiny we need architectures
and emulators supported where native debugging is simple.
Last time stucked with microblaze as only system where
I could run some gdb somehow :(

mk/build.mk

@@ -323,7 +323,7 @@ $(CONFIG)/mconf:
 	@$(MAKE) -C $(CONFIG)
 
 defconfig: .menu $(CONFIG)/conf
-	-@rm $(ADK_TOPDIR)/.defconfig 2>/dev/null
+	@if [ -f $(ADK_TOPDIR)/.defconfig ]; then rm $(ADK_TOPDIR)/.defconfig;fi
 	@if [ ! -z "$(ADK_NO_CHECKSUM)" ];then \
 		echo "ADK_DISABLE_CHECKSUM=y" >> $(ADK_TOPDIR)/.defconfig; \
 	fi

target/config/Config.in.libc

@@ -194,23 +194,29 @@ endchoice
 
 choice
 prompt "Threading"
-depends on ADK_TARGET_LIB_UCLIBC_NG && !ADK_TARGET_WITHOUT_THREADS
+depends on ADK_TARGET_SUPPORTS_THREADS
 
-config ADK_TARGET_LIB_WITH_THREADS
-	bool "enable threads"
+config ADK_TARGET_WITH_NPTL
+	bool "enable NPTL"
+	depends on ADK_TARGET_SUPPORTS_NPTL
+
+config ADK_TARGET_WITH_LT
+	bool "enable Linuxthreads"
+	depends on ADK_TARGET_SUPPORTS_LT && ADK_TARGET_LIB_UCLIBC_NG
 
-config ADK_TARGET_LIB_WITHOUT_THREADS
+config ADK_TARGET_WITHOUT_THREADS
 	bool "disable threads"
+	depends on ADK_TARGET_LIB_UCLIBC_NG
 
 endchoice
 
-config ADK_TARGET_WITH_LT
+config ADK_TARGET_SUPPORTS_THREADS
 	bool
 
-config ADK_TARGET_WITH_NPTL
+config ADK_TARGET_SUPPORTS_NPTL
 	bool
 
-config ADK_TARGET_WITHOUT_THREADS
+config ADK_TARGET_SUPPORTS_LT
 	bool
 
 config ADK_TARGET_LIBC