OvmfPkg: introduce the FD_SIZE_IN_KB macro / build flag

OvmfPkg: introduce the FD_SIZE_IN_KB macro / build flag

FD_SIZE_xMB defines have existed for flash size selection. They can be
passed as "-D FD_SIZE_xMB" on the command line. Passing multiple of them
at the same time has never been supported; earlier settings on the command
line cannot be overridden.

Introduce the integer valued FD_SIZE_IN_KB macro, which provides the
following improvements:

- several instances of it are permitted on the command line, with the last
  one taking effect,

- conditional statements in the DSC and FDF files need only check a single
  macro, and multiple values can be checked in a single !if with the ||
  operator,

- nested !ifdef / !else ladders can be replaced with flat equality tests,

- in the future, flash sizes can be expressed with a finer than MB
  granularity, if necessary.

For now, we're going to preserve the FD_SIZE_xMB defines as convenience
wrappers for FD_SIZE_IN_KB.

FD_SIZE_IN_KB is being added to the DSC files because this way we can
depend on it in both the DSC and FDF files.

Cc: Gary Ching-Pang Lin <glin@suse.com>
Cc: Jordan Justen <jordan.l.justen@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>