efi_loader: use CONFIG_STACK_SIZE in the UEFI sub-system

The Kconfig symbol CONFIG_STACK_SIZE is used both by ARM and Microblaze
with the same meaning. Move it to menu 'General setup' so that we can use
it for all architectures.

Use the value of CONFIG_STACK_SIZE instead of a hard coded 16 MiB value for
reserving memory in the UEFI sub-system.

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