ShellPkg: ShellCommands/SetVar: Make '-rt' imply '-bs'

It's invalid to set a variable that's available from runtime services but not
from boot services.

Currently if you pass '-rt' without '-bs' you get a generic
'Invalid Parameter' message. We should either print a more useful message in
this case, or make '-rt' imply '-bs' (as this patch does). The Shell Spec is
ambiguous on the matter.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Brendan Jackman <Brendan.Jackman@arm.com>
Reviewed-By: Olivier Martin <olivier.martin@arm.com>
Reviewed-By: Jaben Carsey <jaben.carsey@intel.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15323 6f19259b-4bc3-4df7-8a09-765794883524