cpu/cortexm: implement sched_arch_idle() and disable idle thread

cpu/cortexm_common/Kconfig

@@ -13,11 +13,13 @@ config CPU_ARCH_ARMV7M
     bool
     select HAS_ARCH_ARM
     select HAS_ARCH_32BIT
+    select HAS_NO_IDLE_THREAD
 
 config CPU_ARCH_ARMV8M
     bool
     select HAS_ARCH_ARM
     select HAS_ARCH_32BIT
+    select HAS_NO_IDLE_THREAD
 
 config CPU_ARCH
     default "armv6m" if CPU_ARCH_ARMV6M

cpu/cortexm_common/Makefile.features

@@ -30,3 +30,8 @@ else ifeq ($(CPU_CORE),cortex-m23)
 else
   $(error Unkwnown cortexm core: $(CPU_CORE))
 endif
+
+# cortex-m3 and higher don't need the idle thread
+ifneq (,$(filter armv7m armv8m,$(CPU_ARCH)))
+  FEATURES_PROVIDED += no_idle_thread
+endif

cpu/cortexm_common/thread_arch.c

@@ -446,3 +446,24 @@ void __attribute__((used)) isr_svc(void)
     SCB->ICSR = SCB_ICSR_PENDSVSET_Msk;
 }
 #endif /* MODULE_CORTEXM_SVC */
+
+void sched_arch_idle(void)
+{
+    /* by default, PendSV has the same priority as other ISRs.
+     * In this function, we temporarily lower the priority (set higher value),
+     * allowing other ISRs to interrupt.
+     *
+     * According to [this](http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dai0321a/BIHJICIE.html),
+     * dynamically changing the priority is not supported on CortexM0(+).
+     */
+    NVIC_SetPriority(PendSV_IRQn, CPU_CORTEXM_PENDSV_IRQ_PRIO + 1);
+    __DSB();
+    __ISB();
+#ifdef MODULE_PM_LAYERED
+    void pm_set_lowest(void);
+    pm_set_lowest();
+#else
+    __WFI();
+#endif
+    NVIC_SetPriority(PendSV_IRQn, CPU_CORTEXM_PENDSV_IRQ_PRIO);
+}