Cypress: fix flash_area_write, warnings, improve make, readme

Signed-off-by: Bohdan Kovalchuk <bohd@cypress.com>
diff --git a/boot/cypress/README.md b/boot/cypress/README.md
index 1d87605..66f9597 100644
--- a/boot/cypress/README.md
+++ b/boot/cypress/README.md
@@ -17,6 +17,9 @@
 
 The flash map is defined through sysflash.h and cy_flash_map.c.
 
+It is also possible to place secondary (upgrade) slots in external memory module so resulting image size can be doubled.
+For more details about External Memory usage, please refer to separate guiding document `MCUBootApp/ExternalMemory.md`.
+
 MCUBootApp checks image integrity with SHA256, image authenticity with EC256 digital signature verification and uses completely SW implementation of cryptographic functions based on mbedTLS Library.
 
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