configs: earlycon=sbi is deprecated
Console output through SBI has been deprecated. Cf.
https://github.com/riscv/riscv-sbi-doc/blob/master/riscv-sbi.adoc#4-legacy-extensions-eids-0x00---0x0f
Linux with CONFIG_SERIAL_8250_CONSOLE=y has a driver 8250_early.c which
can provide for early output.
Given a DTB with a proper /chosen/stdout-path, plain "earlycon" is
sufficient.
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>