package/swupdate: do not store local build details in swupdate config file

The SWUPDATE_SET_BUILD_OPTIONS macro sets a number of swupdate
configuration options with local build details, especially the
cross-compiler path and sysroot path.

This means that if one stores an swupdate defconfig file as part of
Buildroot, generated with "make swupdate-update-defconfig", it will
contain things like:

CONFIG_CROSS_COMPILE="/home/thomas/projets/buildroot/output/host/bin/arm-linux-"
CONFIG_SYSROOT="/home/thomas/projets/buildroot/output/host/arm-buildroot-linux-uclibcgnueabi/sysroot"

which obviously are not good, as they are specific to where the build
was done.

So instead this commit:

 - Uses the CROSS_COMPILE environment variable to pass the
   cross-compiler path.

 - Drops entirely the use of CONFIG_SYSROOT, since all it does is pass
   a --sysroot option to the compiler, which is not needed in the
   context of Buildroot.

 - Pass EXTRA_CFLAGS/EXTRA_LDFLAGS also through the environment.

Thanks to that the swupdate defconfig file no longer contains any
local build details, and can be re-used by different users of a given
Buildroot configuration.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>