M20 L updated format and first implementation of timestamp reporting (M20 T)

[leptun]

The M20 L format was updated to be in line with the octoprint implementation. Compared to 3.10.0-RC1/2, the size and LFN are swapped. It results in the following output:
M20:

Begin file list
TEST1.GCO 0
LFNFIL~1.GCO 0
End file list

M20 L:

Begin file list
TEST1.GCO 0 "TEST1.GCO"
DIR_ENTER: /TESTFO~1/ "test folder"
DIR_EXIT
LFNFIL~1.GCO 0 "LFN file.gcode"
End file list

The second thing that was fixed in this PR is that M20 L no longer enters and exits system/hidden directories, resulting in a cleaner output.

Timestamps were implemented as well. They can be obtained by issuing M20 T. Both L and T parameters can be used at the same time successfully. The timestamp is a combination of both the date and time into a single integer and printed as a hex.
M20 L T:

Begin file list
TEST1.GCO 0 0x52936b00 "TEST1.GCO"
DIR_ENTER: /TESTFO~1/ "test folder"
DIR_EXIT
LFNFIL~1.GCO 0 0x52936b08 "LFN file.gcode"
End file list

M20 T:

Begin file list
TEST1.GCO 0 0x52936b00
LFNFIL~1.GCO 0 0x52936b08
End file list

This is how the hex is generated:

printf_P(PSTR(" %#lx"), ((uint32_t)crmodDate << 16) | crmodTime);

@todo
Update doxygen
Update the reprap gcode documentation

PFW-1236