Play musical-KNOBS a bit more with release/arm/release.sh.

For stable/10, r264703 sets the correct WITH/WITHOUT
knobs to get xdev built with the arm-freebsd-gcc binary
installed.  Unfortunately, the same fix does not work on
head/.

Also, quite to my amazement, WITH_GCC=1 and WITH_GNUCXX=1
causes xdev to fail spectactularly at least on r264791.

The situation as it stands is:

 - gcc(1) is needed for the u-boot build.
 - cc(1) *cannot* be clang(1)

To shoe-horn the toolchain to make 'xdev' give what is
needed, remove WITH_GNUCXX=1 and add WITH_GCC_BOOTSTRAP=1.

MFC After:	1 week
X-MFC-To:	stable/10 only
X-MFC-Note:	after stable/10 is broken in this way...
Sponsored by:	The FreeBSD Foundation
Move xdev knobs from release/arm/release.sh and into an
XDEV_FLAGS variable in ${KERNCONF}.conf file.

MFC after:	3 days
X-MFC-Note:	fix stable/10 XDEV_FLAGS local for branch
Sponsored by:	The FreeBSD Foundation
Fix grammar error and trailing newline.

Submitted by:	danfe
MFC after:	3 days