[kernel] Make bsp_idle a separate symbol

When debugging, or in panic callstacks, the BSP idle thread used to be
reported as `_kernel_start`, because it was just the loop at the end of
that assembly function. Now, wrap that loop in a separate symbol called
`bsp_idle` to make it clearer that the cpu is in the idle thread.

src/kernel/boot.s

@@ -32,11 +32,14 @@ _kernel_start:
 	extern kernel_main
 	call kernel_main
 
-	; Kernel init is over, wait for the scheduler to
-	; take over
-.hang:
+    ; Kernel init is over, fall through to bsp_idle and wait for
+    ; the scheduler to take over
+.end:
+
+global bsp_idle:function (bsp_idle.end - bsp_idle)
+bsp_idle:
 	hlt
-	jmp .hang
+	jmp bsp_idle
 .end:
 
 global interrupts_enable