BL: Query and store e820 memory map

- bl stage 1: move gdt+pm setup to stage 2 (avoids mode switching)
- bl stage 2: at entry query e820 table from bios, then setup gdt+pm
- kernel/memmap: pad map entry to 24 bytes, as exported by bl
- kernel/memmap: map copy (gbowne1)
- Makefile: facilitate placing the memory map at a known location
- Update bl docs

kernel/memmap.h

@@ -7,6 +7,7 @@ typedef struct {
     uint64_t base_addr;
     uint64_t length;
     uint32_t type;
+    uint32_t ext;
 } __attribute__((packed)) memory_map_entry_t;
 
 uint32_t get_memory_map(memory_map_entry_t *map, uint32_t max_entries);