Add configurable ramstage support for minimal PCI scanning

This CL has changes that allow us to enable a configurable
ramstage, and one change that allows us to minimize PCI
scanning. Minimal scanning is a frequently requested feature.

To enable it, we add two new variables to src/Kconfig
CONFIGURABLE_RAMSTAGE
is the overall variable controlling other options for minimizing the
ramstage.

MINIMAL_PCI_SCANNING
is how we indicate we wish to enable minimal PCI scanning.

Some devices must be scanned in all cases, such as 0:0.0.

To indicate which devices we must scan, we add a new
mandatory
keyword to sconfig

It is used in place of on, off, or hidden, and indicates
a device is enabled and mandatory. Mandatory
devices are always scanned. When MINIMAL_PCI_SCANNING is enabled,
ONLY mandatory devices are scanned.

We further add support in src/device/pci_device.c to manage
both MINIMAL_PCI_SCANNING and mandatory devices.

Finally, to show how this works in practice, we add mandatory
keywords to 3 devices on the qemu-q35.

This is tested and working on the qemu-q35 target with a linux kernel
payload and u-root initramfs.

Change-Id: I2073d9f8e9297c2b02530821ebb634ea2a5c758e
Signed-off-by: Ronald G. Minnich <rminnich@gmail.com>
---
M src/Kconfig
M src/device/pci_device.c
M src/include/device/device.h
M src/mainboard/emulation/qemu-q35/devicetree.cb
M util/sconfig/lex.yy.c_shipped
M util/sconfig/main.c
M util/sconfig/sconfig.h
M util/sconfig/sconfig.l
M util/sconfig/sconfig.tab.c_shipped
M util/sconfig/sconfig.tab.h_shipped
M util/sconfig/sconfig.y
11 files changed, 394 insertions(+), 354 deletions(-)