docs/manual/using-buildroot-debugger: suggest '-ix' iso '-x' when loading gdbinit

docs/manual/using-buildroot-debugger: suggest '-ix' iso '-x' when loading gdbinit

A gdbinit file passed via '-x' will be read _after_ parsing any
object/core file passed on the command-line. In cross-compilation context,
this is particularly a problem when loading a core file, because without the
'sysroot' specified in the gdbinit file, it will give a lot of warnings,
like:

    warning: .dynamic section for "/lib/libstdc++.so.6" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/librt.so.1" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/libm.so.6" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/libgcc_s.so.1" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/libc.so.6" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/ld-linux.so.2" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/libanl.so.1" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/libdl.so.2" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/libpthread.so.0" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/usr/lib/libz.so.1" is not at the expected address (wrong library or version mismatch?)
    warning: .dynamic section for "/lib/libnss_files.so.2" is not at the expected address (wrong library or version mismatch?)

    warning: Could not load shared library symbols for 17 libraries, e.g. [...]
    Use the "info sharedlibrary" command to see the complete listing.
    Do you need "set solib-search-path" or "set sysroot"?

In contrast, the '-ix' option will load the specified gdbinit file _before_
parsing object/core files. This will remove said warnings.

See also: https://sourceware.org/bugzilla/show_bug.cgi?id=28330

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>