ShellPkg: Export default shell delay as PCD

ShellPkg: Export default shell delay as PCD

Create PcdShellDefaultDelay to configure the default
delay the shell provides for the user at the start time
if the user wishes to cancel the execution of a potential
startup script.

The shell application already allows the user to override
the delay default value by specifying the -delay cmdline
argument. This however cannot be used when loading the
shell application using direct boot or when integrating
the shell into the platform firmware build.

Thus, a PCD can be easily configured by the developer
either at build time, or even at runtime.

Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Tomas Pilar <tomas@quicinc.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>