normal: Move common datetime functions out of the normal module

The common datetime helper functions are currently included in the normal
module, but this makes any other module that calls these functions to have
a dependency with the normal module only for this reason.

Since the normal module does a lot of stuff, it calls functions from other
modules. But since other modules may depend on it for calling the datetime
helpers, this could lead to circular dependencies between modules.

As an example, when platform == xen the grub_get_datetime() function from
the datetime module calls to the grub_unixtime2datetime() helper function
from the normal module. Which leads to the following module dependency:

    datetime -> normal

and send_dhcp_packet() from the net module calls the grub_get_datetime()
function, which leads to the following module dependency:

    net -> datetime -> normal

but that means that the normal module is not allowed to depend on net or
any other module that depends on it due the transitive dependency caused
by datetime. A recent patch attempted to add support to fetch the config
file over the network, which leads to the following circular dependency:

    normal -> net -> datetime -> normal

So having the datetime helpers in the normal module makes it quite fragile
and easy to add circular dependencies like these, that break the build due
the genmoddep.awk script catching the issues.

Fix this by taking the datetime helper functions out of the normal module
and instead add them to the datetime module itself. Besides fixing these
issues, it makes more sense to have these helper functions there anyways.

Reported-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

Makefile.util.def

@@ -141,7 +141,7 @@ library = {
   common = grub-core/lib/crc.c;
   common = grub-core/lib/adler32.c;
   common = grub-core/lib/crc64.c;
-  common = grub-core/normal/datetime.c;
+  common = grub-core/lib/datetime.c;
   common = grub-core/normal/misc.c;
   common = grub-core/partmap/acorn.c;
   common = grub-core/partmap/amiga.c;

grub-core/Makefile.core.def

@@ -1687,6 +1687,7 @@ module = {
 
 module = {
   name = datetime;
+  common = lib/datetime.c;
   cmos = lib/cmos_datetime.c;
   efi = lib/efi/datetime.c;
   uboot = lib/dummy/datetime.c;
@@ -1926,7 +1927,6 @@ module = {
   common = normal/autofs.c;
   common = normal/color.c;
   common = normal/completion.c;
-  common = normal/datetime.c;
   common = normal/menu.c;
   common = normal/menu_entry.c;
   common = normal/menu_text.c;