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>

 # PPL options

 choice

     bool

     prompt "PPL version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config PPL_V_0_11

     bool

     prompt "0.11 (EXPERIMENTAL)"

     depends on EXPERIMENTAL

 config PPL_V_0_10_2

     bool

     prompt "0.10.2"

 endchoice

 config PPL_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "0.11" if PPL_V_0_11

     default "0.10.2" if PPL_V_0_10_2