esp32/boards/sdkconfig.base: Resize SSL output buffer from 16 to 4kiB.

The IDF heap is more fragmented with IDF 4 and mbedtls cannot allocate
enough RAM with 16+16kiB for both in and out buffers, so reduce output
buffer size.

Fixes issue #5303.

ports/esp32/boards/sdkconfig.base

@@ -28,6 +28,10 @@ CONFIG_LWIP_PPP_SUPPORT=y
 CONFIG_LWIP_PPP_PAP_SUPPORT=y
 CONFIG_LWIP_PPP_CHAP_SUPPORT=y
 
+# SSL
+# Use 4kiB output buffer instead of default 16kiB (because IDF heap is fragmented in 4.0)
+CONFIG_MBEDTLS_ASYMMETRIC_CONTENT_LEN=y
+
 # v3.3-only (renamed in 4.0)
 CONFIG_LOG_BOOTLOADER_LEVEL_WARN=y
 CONFIG_TASK_WDT_CHECK_IDLE_TASK_CPU0=n