[S390] kernel: Storing machine flags early in lowcore

Currently, the machine_flags are stored late in the startup
initialization which results in failing machine type checks
(e.g. for MACHINE_IS_VM).
To allow these checks, store the machine flags in the lowcore
when the machine type has been detected.

Moving the machine_flags to the lowcore has been introduced with
git commit 25097bf1

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>