target/riscv: Add short-isa-string option

target/riscv: Add short-isa-string option

Because some operating systems don't correctly parse long ISA extension
string, this commit adds short-isa-string boolean option to disable
generating long ISA extension strings on Device Tree.

For instance, enabling Zfinx and Zdinx extensions and booting Linux (5.17 or
earlier) with FPU support caused a kernel panic.

Operating Systems which short-isa-string might be helpful:

1.  Linux (5.17 or earlier)
2.  FreeBSD (at least 14.0-CURRENT)
3.  OpenBSD (at least current development version)

Signed-off-by: Tsukasa OI <research_trasio@irq.a4lg.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <7c1fe5f06b0a7646a47e9bcdddb1042bb60c69c8.1652181972.git.research_trasio@irq.a4lg.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>