Makefile: help: BR2_DEFCONFIG for defconfig must be on command line

Makefile: help: BR2_DEFCONFIG for defconfig must be on command line

The help text says that BR2_DEFCONFIG will be used as input, but a
BR2_DEFCONFIG specified in the existing .config file will *not* be
used. So say explicitly that it must be specified on the command line.
Note that both "BR2_DEFCONFIG=... make defconfig" and
"make defconfig BR2_DEFCONFIG=..." will work.

While we're at it, add a semicolon to separate the two statements.

Note that this overflows the help text beyond 80 characters, but that
is already the case in many other lines.

Reported-by: Anisse Astier <anisse@astier.eu>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>