toolchain: mark sysroot as 'safe' path for gdb auto-load

toolchain: mark sysroot as 'safe' path for gdb auto-load

gdb can automatically load certain files as described in [1]. Such files
could install pretty-printers for complex data structures.

libstdcxx (C++ standard library) provided by gcc, is one example of a
library for which such auto-load file is available. But there are other
examples too, like libglib2.

However, gdb will only auto-load files if the file is located in one of the
locations treated as 'safe'. The Buildroot sysroot is not by default in that
list.

Provide a better debugging experience by adding the sysroot to the 'safe'
list, via the gdbinit file prepared by Buildroot.

[1] https://sourceware.org/gdb/onlinedocs/gdb/objfile_002dgdbdotext-file.html

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>