config: add an option to name the sysroot directory

Depending on local policies, some users have expressed a need to
have the sysroot be named differently than the hard-coded name.

Add an option for that.
Default to 'sysroot' to match the existing literature.

While at it, replace 'sys-root' with 'sysroot' everywhere we
reference the sysroot.

Reported-by: Alexey Kuznetsov <Alexey.KUZNETSOV@youtransactor.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

# Super-H specific configuration file

choice

    bool

    prompt "Variant"

config ARCH_SH_SH3

    bool

    prompt "sh3"

config ARCH_SH_SH4

    bool

    prompt "sh4"

config ARCH_SH_SH4A

    bool

    prompt "sh4a"

endchoice

config ARCH_SH_VARIANT

    string

    default "sh3"   if ARCH_SH_SH3

    default "sh4"   if ARCH_SH_SH4

    default "sh4a"  if ARCH_SH_SH4A