STM32: adjust flash.get_page_size() to minimum programable size

Users of FlashIAP usually get the minimum programable size
by calling flash.get_page_size(), so let's return the minimum
to allows a most efficient usage of flash.

For F4 devices, this is 1 byte.
For L0 and L1 devices, this is a word (4 bytes).
For L4 devices, this is a double word (8 bytes).

targets/TARGET_STM/TARGET_STM32F4/flash_api.c

@@ -129,9 +129,10 @@ uint32_t flash_get_sector_size(const flash_t *obj, uint32_t address)
 
 uint32_t flash_get_page_size(const flash_t *obj)
 {
-    // not applicable for STM32F4
-    return (0x4000); // minimum sector size
+    // Flash of STM32F4 devices can be programed 1 byte at a time
+    return (1);
 }
+
 uint32_t flash_get_start_address(const flash_t *obj)
 {
     return FLASH_BASE;

targets/TARGET_STM/TARGET_STM32L0/flash_api.c

@@ -128,7 +128,8 @@ uint32_t flash_get_sector_size(const flash_t *obj, uint32_t address) {
 }
 
 uint32_t flash_get_page_size(const flash_t *obj) {
-    return FLASH_PAGE_SIZE;
+    /*  Page size is the minimum programable size, which 4 bytes */
+    return 4;
 }
 
 uint32_t flash_get_start_address(const flash_t *obj) {

targets/TARGET_STM/TARGET_STM32L1/flash_api.c

@@ -127,7 +127,8 @@ uint32_t flash_get_sector_size(const flash_t *obj, uint32_t address)
 
 uint32_t flash_get_page_size(const flash_t *obj)
 {
-    return FLASH_PAGE_SIZE;
+    /*  Page size is the minimum programable size, which 4 bytes */
+    return 4;
 }
 
 uint32_t flash_get_start_address(const flash_t *obj) 

targets/TARGET_STM/TARGET_STM32L4/flash_api.c

@@ -221,8 +221,8 @@ uint32_t flash_get_sector_size(const flash_t *obj, uint32_t address) {
  * @return The size of a page
  */
 uint32_t flash_get_page_size(const flash_t *obj) {
-    /*  considering 1 sector = 1 page */
-    return FLASH_PAGE_SIZE;
+    /*  Page size is the minimum programable size, which 8 bytes */
+    return 8;
 }
 
 /** Get start address for the flash region