Add lesson 21

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+Let's create a another variable `PcdMyVar32_1` the same way we did in the previous lesson.
+
+Add a PCD definition to the `UefiLessonsPkg/UefiLessonsPkg.dec`:
+```
+[PcdsFixedAtBuild]
+  ...
+  gUefiLessonsPkgTokenSpaceGuid.PcdMyVar32_1|42|UINT32|0x00000002
+```
+
+Add print statement to `UefiLessonsPkg/PCDLesson/PCDLesson.c`:
+```
+Print(L"PcdMyVar32_1=%d\n", FixedPcdGet32(PcdMyVar32_1));
+```
+
+As for the `PCDLesson.inf` this time add an override for a value this time:
+```
+[FixedPcd]
+  ...
+  gUefiLessonsPkgTokenSpaceGuid.PcdMyVar32_1|43
+```
+
+If you build execute our app under OVMF you would get:
+```
+FS0:\> PCDLesson.efi
+PcdMyVar32=42
+PcdMyVar32_1=43
+```
+
+So it means that every App/Driver can override a PCD declared in *.dec file differently.
+
+______________
+
+Now let's create a third variable `PcdMyVar32_2` the same way as `PcdMyVar32_1`.
+`UefiLessonsPkg/UefiLessonsPkg.dec`
+ ```
+[PcdsFixedAtBuild]
+  ...
+  gUefiLessonsPkgTokenSpaceGuid.PcdMyVar32_2|42|UINT32|0x00000003
+```
+UefiLessonsPkg/PCDLesson/PCDLesson.c
+```
+Print(L"PcdMyVar32_2=%d\n", FixedPcdGet32(PcdMyVar32_2));
+```
+UefiLessonsPkg/PCDLesson/PCDLesson.inf
+```
+[FixedPcd]
+  ...
+  gUefiLessonsPkgTokenSpaceGuid.PcdMyVar32_2|43
+```
+Only this time add an override for the variable to our `UefiLessonsPkg/UefiLessonsPkg.dsc` file:
+```
+[PcdsFixedAtBuild]
+  gUefiLessonsPkgTokenSpaceGuid.PcdMyVar32_2|44
+```
+
+If you build our app and execute in under OVMF now you would get:
+```
+FS0:\> PCDLesson.efi
+PcdMyVar32=42
+PcdMyVar32_1=43
+PcdMyVar32_2=44
+```
+
+So override order would be:
+```
+DEC < INF < DSC
+```
+- Declaration file (DEC) declares PCD with its default value
+- Every App/Driver Information file (INF) can override the value of this PCD differently
+- However a package description file that contains all these Apps/Drivers (DSC) can override this PCD for all of them
+