[boot] Fix several errors getting to kernel

* When using the non-allocating version of `get_uefi_mappings` the
  length was not getting set. Reworked this function.
* Having `build_kernel_mem_map` from `bootloader_main_uefi` caused it to
  get an out of date map key. Moved this function into `efi_main` right
  before exiting boot services.

src/boot/memory.h

@@ -80,11 +80,6 @@ struct efi_mem_map
 	iterator end() { return offset_ptr<desc>(entries, length); }
 };
 
-/// Get the memory map from UEFI.
-/// \arg allocate  If false, don't actually fetch the mappings, just
-///                return a structure describing them.
-efi_mem_map get_uefi_mappings(bool allocate, uefi::boot_services *bs);
-
 /// Add the kernel's memory map as a module to the kernel args.
 /// \returns  The uefi memory map key for the version used to build
 ///           this map