efi_loader: ResetSystem() should not hang

If ResetSystem() is not implemented at runtime, it should return instead
of hanging in an endless loop. This allows the operating system to reset
the system by other means as Linux does. It also matches what EDK II
suggests in comments for functions ResetShutdown() and ResetWarm() in
OvmfPkg/Library/ResetSystemLib/ResetSystemLib.c.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>