configs: fix build of calao_usb_a9g20_lpw_defconfig

Both the Barebox and AT91Bootstrap patches for the Calao USB A9G20
board are located in the same directory, board/calao/usb-a9g20-lpw/,
with only a prefix making a difference between whether they apply to
Barebox or AT91Bootstrap.

Unfortunately, since commit 0eba4759
("packages: apply custom patches using *.patch instead of
<pkg>-*.patch"), the logic to apply custom patches in Barebox and
AT91Bootstrap has been changed from applying <package>-*.patch to
applying *.patch (in order to be consistent with what we do for all
packages in Buildroot).

However, this had the effect of trying to apply both the AT91Bootstrap
and Barebox patches on both packages, which obviously cannot work.

This commit fixes that by moving patches around, and switching to use
BR2_GLOBAL_PATCH_DIR.

Cc: Gregory Hermant <gregory.hermant@calao-systems.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>