debug/gdb: fix runtime sysroot

This patch sets the runtime sysroot to fix the following GDB warning:
"Unable to find dynamic linker breakpoint function.
GDB will be unable to debug shared library initializers
and track explicitly loaded dynamic code."

The sysroot can later be changed within gdb with the `set sysroot`
command if necessary.

Signed-off-by: "Benoît THÉBAUDEAU" <benoit.thebaudeau@advansee.com>

 # MIPS specific config options

 ## select ARCH_SUPPORTS_32

 ## select ARCH_SUPPORTS_64 if EXPERIMENTAL

 ## select ARCH_DEFAULT_32

 ## select ARCH_USE_MMU

 ## select ARCH_SUPPORTS_BOTH_ENDIAN

 ## select ARCH_DEFAULT_BE

 ## select ARCH_SUPPORT_ARCH

 ## select ARCH_SUPPORT_TUNE

 ##

 ## help The MIPS architecture, as defined by:

 ## help     http://www.mips.com/