Update README with new mkuimage instructions

Signed-off-by: Chris Koch <[email protected]>

Advertise the supported boot commands

Signed-off-by: Chris Koch <[email protected]>

Make gosh default shell

Signed-off-by: Chris Koch <[email protected]>

spidev: improve regexp for testing

Signed-off-by: Ronald G. Minnich <[email protected]>

bump to v-1.11.0

The major driver for this was that v0.10.0 broke tamago armoryboot.

But it makes sense to do it anyway, with all the bug fixes and cleanups
that happened recently.

Signed-off-by: Ronald G. Minnich <[email protected]>

bump version to 0.10.0

Create v0.9.0 release notes

Signed-off-by: Ryan O'Leary <[email protected]>

go.mod: add +incompatible to retractions

go mod tidy requires this. This also allows us to in the future release
versions v2 through v7 if we ever desire to.

See also:

```
$ go build
go: errors parsing go.mod:
go.mod:67:2: retract github.com/u-root/u-root: version "v2.0.0" invalid: should be v0 or v1, not v2
go.mod:69:2: retract github.com/u-root/u-root: version "v3.0.0" invalid: should be v0 or v1, not v3
go.mod:71:2: retract github.com/u-root/u-root: version "v4.0.0" invalid: should be v0 or v1, not v4
go.mod:73:2: retract github.com/u-root/u-root: version "v5.0.0" invalid: should be v0 or v1, not v5
go.mod:75:2: retract github.com/u-root/u-root: version "v6.0.0" invalid: should be v0 or v1, not v6
$ go mod edit -retract=v2.0.0
go mod: -retract=v2.0.0: version "v2.0.0" invalid: should be v2.0.0+incompatible (or module github.com/u-root/u-root/v2)
```

Signed-off-by: Chris Koch <[email protected]>

Restructure main

Add more tests

Signed-off-by: Christopher Meis <[email protected]>

boot: support CatInitrds in ipxe

Enable to concat initrd files natively, similar to what was done earlier
in syslinux.go.

Signed-off-by: Robi Buranyi <[email protected]>