segmentless smp fixes

adjust the smp booting procedure for segmentless operation. changes are
mostly due to gdt/idt being dependent on paging, because of the high
location, and paging being on much sooner because of that too.

also smaller fixes: redefine DESC_SIZE, fix kernel makefile variable name
(crosscompiling), some null pointer checks that trap now because of a
sparser pagetable, acpi sanity checking

kernel/main.c

@@ -107,6 +107,9 @@ void bsp_finish_booting(void)
   machine.bsp_id = 0;
 #endif
 
+  /* Kernel may no longer use bits of memory as VM will be running soon */
+  kernel_may_alloc = 0;
+
   switch_to_user();
   NOT_REACHABLE;
 }
@@ -128,6 +131,9 @@ void kmain(kinfo_t *local_cbi)
   /* We can talk now */
   printf("MINIX booting\n");
 
+  /* Kernel may use bits of main memory before VM is started */
+  kernel_may_alloc = 1;
+
   assert(sizeof(kinfo.boot_procs) == sizeof(image));
   memcpy(kinfo.boot_procs, image, sizeof(kinfo.boot_procs));