Update ipxe version

This is a tentative to make ipxe more close to the current version
and include some improvements and bug fixes.
As a first step we build and install ipxe using a commit hash,
as no stable versions have been released since December 2020.

We point the ipxe commit hash to [1] from November 2021, so
roughly a year of changes is included.
To see the complete list of changes run:
`git log --pretty=oneline 988d2c1..9062544`
from a local clone of the ipxe repository.

In general the changes included between the stable 1.21.1
version and the current chosen hash improve compatibility
with recent gcc and build libraries, while fixing
numerous bugs.

This change also introduce a build arg to allow choosing
the ipxe commit hash at container build time.

[1] ipxe/ipxe@9062544

Signed-off-by: Riccardo Pittau <elfosardo@gmail.com>