espressif: use minimal libc as default for ESP32 boards

Make MINIMAL_LIBC as default for MCUboot app build instead
of picolibc. Footprint is lower and no need to
MULTLTHREADING enabled for SoC build.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>
diff --git a/boot/zephyr/boards/esp32s2_franzininho.conf b/boot/zephyr/boards/esp32s2_franzininho.conf
index 70a092a..b2b4059 100644
--- a/boot/zephyr/boards/esp32s2_franzininho.conf
+++ b/boot/zephyr/boards/esp32s2_franzininho.conf
@@ -16,3 +16,4 @@
 CONFIG_DEBUG=n
 
 CONFIG_HEAP_MEM_POOL_SIZE=4096
+CONFIG_MINIMAL_LIBC=y