Changes

contrib/jenkins: add SKIP_FIRMWARE_CLEAN argument

If this is set, then don't clean the firmware at the end. This is needed
to store the built firmware as artifact in jenkins, for passing it along
to another job.

Related: SYS#7963
Change-Id: I9e2342b8111645ef5d3b7bf4ac364ca592aa600b

firmware: werror on missing return

Silently ignoring a missing return just broke all timers,
so prevent similar mistakes in the future.

Change-Id: I2e25884077af6334c9e9ddace3900b04061fae04

firmware: fix timers, add missing return

Change-Id: I5b12284c88d8fe6e4fe55cc4cd2aad550d276af4

firmware: add tear-free 64bit load/store helpers for jiffies

On Cortex-M4 a 64bit load may compile to two separate LDR
instructions which can lead to torn reads due to interrupts.
LDRD/STRD are restartable, they either complete or restart from
scratch, so the result is always consistent. The only "downside" is the
required alignment, which is fine.

Change-Id: I729c0fdfb5b228b03c2df1cf098743100b1ea625

firmware: replace libosmocore fsm timers

Concurrent access from main loop and the different uart irqs was not
going well and caused crashes. The libosmocore fsm does still allocate
one internal timer, but it is not used by the firmware, so libosmocore
will not touch the rbtree.

Additionally ignore checkpatch jiffies complaints because we're trying
to do firmware here.

Closes: SYS#7877
Change-Id: Id2bd67b3946bb451008965f0b68b4a919f4d10bd