[nrf noup] boot: add 'minimal' configuration files

Add prj_minimal.conf, a Kconfig fragment to be used for minimally
sized image production. The minimal fragment has been simplified for
only external crypto.

Move partition sizing into Kconfig to be consistent with the method
used by b0.

Using this fragment with prj_minimal.conf makes MCUboot < 16kB for
all nRF devices (9160 still needs 32kB partition).

Ref: NCSDK-6704
Signed-off-by: Stephen Stauts <stephen.stauts@nordicsemi.no>
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit 0701661)

boot/zephyr/boards/nrf5340dk_nrf5340_cpuapp_minimal.conf

@@ -0,0 +1,13 @@
+#
+# Copyright (c) 2021 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+#
+
+# CC3xx is currently not used for nrf53
+CONFIG_HW_CC3XX=n
+CONFIG_NRF_CC3XX_PLATFORM=n
+
+# Required for kernel operation
+CONFIG_CLOCK_CONTROL=y
+CONFIG_SYS_CLOCK_EXISTS=y

boot/zephyr/prj_minimal.conf

@@ -0,0 +1,45 @@
+#
+# Copyright (c) 2021 Nordic Semiconductor ASA
+#
+# SPDX-License-Identifier: LicenseRef-Nordic-5-Clause
+#
+
+CONFIG_MAIN_STACK_SIZE=10240
+CONFIG_MBEDTLS_CFG_FILE="mcuboot-mbedtls-cfg.h"
+
+CONFIG_FLASH=y
+CONFIG_FPROTECT=y
+CONFIG_PM=n
+
+CONFIG_BOOT_ENCRYPT_EC256=n
+CONFIG_BOOT_ENCRYPT_RSA=n
+CONFIG_BOOT_ENCRYPT_X25519=n
+CONFIG_BOOT_SWAP_SAVE_ENCTLV=n
+
+CONFIG_BOOT_BOOTSTRAP=n
+CONFIG_BOOT_UPGRADE_ONLY=n
+
+### Minimal Configurations ###
+CONFIG_BOOT_USE_MIN_PARTITION_SIZE=y
+CONFIG_ASSERT=n
+CONFIG_BOOT_BANNER=n
+CONFIG_CLOCK_CONTROL=n
+CONFIG_CONSOLE=n
+CONFIG_CONSOLE_HANDLER=n
+CONFIG_GPIO=n
+CONFIG_KERNEL_MEM_POOL=n
+CONFIG_LOG=n
+CONFIG_MINIMAL_LIBC_CALLOC=n
+CONFIG_MINIMAL_LIBC_MALLOC=n
+CONFIG_MINIMAL_LIBC_REALLOCARRAY=n
+CONFIG_NCS_SAMPLES_DEFAULTS=n
+CONFIG_NO_RUNTIME_CHECKS=y
+CONFIG_NRF_RTC_TIMER=n
+CONFIG_PRINTK=n
+CONFIG_REBOOT=n
+CONFIG_RESET_ON_FATAL_ERROR=n
+CONFIG_SECURE_BOOT_DEBUG=n
+CONFIG_SERIAL=n
+CONFIG_SIZE_OPTIMIZATIONS=y
+CONFIG_SYS_CLOCK_EXISTS=n
+CONFIG_UART_CONSOLE=n