CONFIG_HEXDUMP is needed to display UEFI variables using 'printenv -e'.

Enabling CONFIG_SPL_HEXDUMP only makes sense for debugging purposes.
Hence CONFIG_SPL_HEXDUMP should not be enabled by default.

The following boards currently have CONFIG_SPL_HEXDUMP=y. This includes
boards that don't use SPL at all.

axm_defconfig
imx8mm-cl-iot-gate_defconfig
imx8mm_venice_defconfig
imxrt1020-evk_defconfig
imxrt1050-evk_defconfig
kontron_sl28_defconfig
kp_imx53_defconfig
lx2160ardb_tfa_stmm_defconfig
mt7622_rfb_defconfig
octeon_ebb7304_defconfig
octeon_nic23_defconfig
qemu_arm64_defconfig
qemu_arm_defconfig
qemu-riscv32_defconfig
qemu-riscv32_smode_defconfig
qemu-riscv64_defconfig
qemu-riscv64_smode_defconfig
qemu-x86_64_defconfig
qemu-x86_defconfig
sandbox64_defconfig
sandbox_defconfig
stm32mp15_basic_defconfig
stm32mp15_trusted_defconfig
synquacer_developerbox_defconfig
taurus_defconfig
xilinx_versal_virt_defconfig

The patch only keeps it enabled on

sandbox_spl_defconfig

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>