Move Kmain into its own package

This allows us to keep the error definition in the kernel package
without causing circular import errors

kernel/kmain/kmain.go

@@ -0,0 +1,27 @@
+package kmain
+
+import (
+	"github.com/achilleasa/gopher-os/kernel/hal"
+	"github.com/achilleasa/gopher-os/kernel/hal/multiboot"
+	"github.com/achilleasa/gopher-os/kernel/mem/pmm"
+)
+
+// Kmain is the only Go symbol that is visible (exported) from the rt0 initialization
+// code. This function is invoked by the rt0 assembly code after setting up the GDT
+// and setting up a a minimal g0 struct that allows Go code using the 4K stack
+// allocated by the assembly code.
+//
+// The rt0 code passes the address of the multiboot info payload provided by the
+// bootloader.
+//
+// Kmain is not expected to return. If it does, the rt0 code will halt the CPU.
+//
+//go:noinline
+func Kmain(multibootInfoPtr uintptr) {
+	multiboot.SetInfoPtr(multibootInfoPtr)
+
+	hal.InitTerminal()
+	hal.ActiveTerminal.Clear()
+
+	pmm.EarlyAllocator.Init()
+}