armcm_timer: Unify udelay() implementation

Move the udelay() code from various arm board directories into the
src/generic/armcm_timer.c code.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

src/generic/armcm_timer.c

@@ -54,6 +54,20 @@ timer_kick(void)
     SCB->ICSR = SCB_ICSR_PENDSTSET_Msk;
 }
 
+// Implement simple early-boot delay mechanism
+void
+udelay(uint32_t usecs)
+{
+    if (!(CoreDebug->DEMCR & CoreDebug_DEMCR_TRCENA_Msk)) {
+        CoreDebug->DEMCR |= CoreDebug_DEMCR_TRCENA_Msk;
+        DWT->CTRL |= DWT_CTRL_CYCCNTENA_Msk;
+    }
+
+    uint32_t end = timer_read_time() + timer_from_us(usecs);
+    while (timer_is_before(timer_read_time(), end))
+        ;
+}
+
 void
 timer_init(void)
 {

src/generic/armcm_timer.h

@@ -0,0 +1,8 @@
+#ifndef __GENERIC_ARMCM_TIMER_H
+#define __GENERIC_ARMCM_TIMER_H
+
+#include <stdint.h> // uint32_t
+
+void udelay(uint32_t usecs);
+
+#endif // serial_irq.h