OvmfPkg/Sec: Support SECTION2 DXEFV types

OvmfPkg/Sec: Support SECTION2 DXEFV types

Support down-stream projects that require large DXEFV sizes greater
than 16MB by handling SECTION2 common headers. These are already
created by the build tools when necessary.

Use IS_SECTION2 and SECTION2_SIZE macros to calculate accurate image
sizes when appropriate.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Thomas Palmer <thomas.palmer@hpe.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Regression-tested-by: Laszlo Ersek <lersek@redhat.com>
[lersek@redhat.com: fix NB->MB typo in commit message]
Signed-off-by: Laszlo Ersek <lersek@redhat.com>