Tweak Rasperry Pi enviroment to avoid broken macro expansion

The Raspberry Pi environment appears to now be being automatically
escaped, however we are already manually escaping in places that results
in unexpected double escaping and breakage during macro expansion:

[ 1979s] include/generated/environment.h:1:296: error: expected ‘}’ before ‘extlinux’
[ 1979s]     1 | #define CONFIG_EXTRA_ENV_TEXT ...   ...boot_syslinux_conf \\"extlinux/extlinux...
[ 1979s]       |                                                              ^~~~~~~~

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>