kernel: qca-ssdk: use bash as shell

kernel: qca-ssdk: use bash as shell

Currently, trying to compile qca-ssdk on macOS will fail in a weird way:
make[6]: *** No rule to make target 'openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-qualcommax_ipq807x/qca-ssdk-2024.04.17~3d060f7a/-n',
needed by 'openwrt/build_dir/target-aarch64_cortex-a53_musl/linux-qualcommax_ipq807x/qca-ssdk-2024.04.17~3d060f7a/qca-ssdk.o'. Stop.

After looking looking at src_list.dep from which KBuild cmd_mod will
generate the list of objects to compile it looked like:
-n /src/adpt/adpt.c
-n
-n

Which was rather suspicous so after comparing to the same file but with
Fedora as host:
/src/adpt/adpt.c src/adpt/hppe/adpt_hppe_fdb.c src/adpt/hppe/adpt_hppe_mib.c

It was clear that echo -n which was used in SSDK-s target.mk was not
working as intented, and it looked like the POSIX only version of echo
was being used which does not honor -n.

So, after failling to reproduce it externally, replacing the call to echo
with a full path to coreutils echo fixed the compilation.

After further debugging, it was determined that SSDK does not honor
CONFIG_SHELL like other kernel modules so it was defaulting to /bin/sh as
the shell make was calling thus calling the /bin/sh built-in echo which on
macOS is the old Bash 3.2 one and it does not respect -n.

So, we have to explicitly pass SHELL=$(BASH) to SSDK to make it use bash
like kernel build or other kernel modules.

This is not an issue since on macOS we always build bash anyway.

Link: https://github.com/openwrt/openwrt/pull/15459
Signed-off-by: Robert Marko <robimarko@gmail.com>